Make a change to prevent false positive background app starts. (#7274)

Added a change in `AppStartTrace` to prevent false positives of a
background app start on API 34+ devices.

See #5920

Verified locally:

```
2025-08-20 10:34:51.283 18373-18373 AppMonDemo              com.tdeshpande.appmontester          D  mainThreadRunnable.run(): activityCreated = false
2025-08-20 10:34:51.292 18373-18373 AppMonDemo              com.tdeshpande.appmontester          D  onActivityPreCreated: mainThreadRunnableRun = true
2025-08-20 10:34:51.294 18373-18373 AppMonDemo              com.tdeshpande.appmontester          D  onActivityCreated: mainThreadRunnableRun = true
2025-08-20 10:34:51.314 18373-18373 AppMonDemo              com.tdeshpande.appmontester          D  onActivityStarted: mainThreadRunnableRun = true
2025-08-20 10:34:51.318 18373-18373 FirebasePerformance     com.tdeshpande.appmontester          I  Firebase Performance Monitoring is successfully initialized! In a minute, visit the Firebase console to view your data: 
2025-08-20 10:34:51.319 18373-18373 FirebasePerformance     com.tdeshpande.appmontester          D  onResume(): com.tdeshpande.appmontester.MainActivity: 74362 microseconds
2025-08-20 10:34:51.320 18373-18373 AppMonDemo              com.tdeshpande.appmontester          D  onActivityResumed: mainThreadRunnableRun = true
2025-08-20 10:34:51.323 18373-18431 FirebasePerformance     com.tdeshpande.appmontester          I  Logging trace metric: _as (duration: 74.362ms). In a minute, visit the Firebase console to view your data: 
```

Author: tejasd

firebase-perf/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 * [fixed] Fixed an ANR on app launch. [#4831]
+* [fixed] Fixed app start traces on API 34+. [#5920]
 
 # 22.0.0
 * [changed] **Breaking Change**: Updated minSdkVersion to API level 23 or higher.

firebase-perf/src/main/java/com/google/firebase/perf/metrics/AppStartTrace.java

@@ -75,6 +75,8 @@ public class AppStartTrace implements ActivityLifecycleCallbacks, LifecycleObser
   private static final @NonNull Timer PERF_CLASS_LOAD_TIME = new Clock().getTime();
   private static final long MAX_LATENCY_BEFORE_UI_INIT = TimeUnit.MINUTES.toMicros(1);
 
+  private static final long MAX_BACKGROUND_RUNNABLE_DELAY = TimeUnit.MILLISECONDS.toMicros(100);
+
   // Core pool size 0 allows threads to shut down if they're idle
   private static final int CORE_POOL_SIZE = 0;
   private static final int MAX_POOL_SIZE = 1; // Only need single thread
@@ -111,6 +113,8 @@ public class AppStartTrace implements ActivityLifecycleCallbacks, LifecycleObser
   private final @Nullable Timer processStartTime;
   private final @Nullable Timer firebaseClassLoadTime;
   private Timer onCreateTime = null;
+
+  private Timer mainThreadRunnableTime = null;
   private Timer onStartTime = null;
   private Timer onResumeTime = null;
   private Timer firstForegroundTime = null;
@@ -319,8 +323,26 @@ private void recordOnDrawFrontOfQueue() {
     logExperimentTrace(this.experimentTtid);
   }
 
+  private void resolveIsStartedFromBackground() {
+    // If the mainThreadRunnableTime is null, either the runnable hasn't run, or this check has
+    // already been made.
+    if (mainThreadRunnableTime == null) {
+      return;
+    }
+
+    // Set it to true if the runnable ran more than 100ms prior to onActivityCreated()
+    if (mainThreadRunnableTime.getDurationMicros() > MAX_BACKGROUND_RUNNABLE_DELAY) {
+      isStartedFromBackground = true;
+    }
+
+    // Set this to null to prevent additional checks if `onActivityCreated()` is called again.
+    mainThreadRunnableTime = null;
+  }
+
   @Override
   public synchronized void onActivityCreated(Activity activity, Bundle savedInstanceState) {
+    resolveIsStartedFromBackground();
+
     if (isStartedFromBackground || onCreateTime != null // An activity already called onCreate()
     ) {
       return;
@@ -560,8 +582,7 @@ public static boolean isScreenOn(Context appContext) {
    * We use StartFromBackgroundRunnable to detect if app is started from background or foreground.
    * If app is started from background, we do not generate AppStart trace. This runnable is posted
    * to main UI thread from FirebasePerfEarly. If app is started from background, this runnable will
-   * be executed before any activity's onCreate() method. If app is started from foreground,
-   * activity's onCreate() method is executed before this runnable.
+   * be executed earlier than 100ms of any activity's onCreate() method.
    */
   public static class StartFromBackgroundRunnable implements Runnable {
     private final AppStartTrace trace;
@@ -572,10 +593,7 @@ public StartFromBackgroundRunnable(final AppStartTrace trace) {
 
     @Override
     public void run() {
-      // if no activity has ever been created.
-      if (trace.onCreateTime == null) {
-        trace.isStartedFromBackground = true;
-      }
+      trace.mainThreadRunnableTime = new Timer();
     }
   }
 
@@ -614,7 +632,7 @@ Timer getOnResumeTime() {
   }
 
   @VisibleForTesting
-  void setIsStartFromBackground() {
-    isStartedFromBackground = true;
+  void setMainThreadRunnableTime(Timer timer) {
+    mainThreadRunnableTime = timer;
   }
 }

firebase-perf/src/test/java/com/google/firebase/perf/metrics/AppStartTraceTest.java

@@ -18,6 +18,7 @@
 import static org.mockito.ArgumentMatchers.isA;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -238,11 +239,42 @@ public void testDelayedAppStart() {
   }
 
   @Test
-  public void testStartFromBackground() {
+  public void testStartFromBackground_within100ms() {
     FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
+    Timer fakeTimer = spy(new Timer(currentTime));
     AppStartTrace trace =
         new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
-    trace.setIsStartFromBackground();
+    trace.registerActivityLifecycleCallbacks(appContext);
+    trace.setMainThreadRunnableTime(fakeTimer);
+
+    when(fakeTimer.getDurationMicros()).thenReturn(99L);
+    trace.onActivityCreated(activity1, bundle);
+    Assert.assertNotNull(trace.getOnCreateTime());
+    ++currentTime;
+    trace.onActivityStarted(activity1);
+    Assert.assertNotNull(trace.getOnStartTime());
+    ++currentTime;
+    trace.onActivityResumed(activity1);
+    Assert.assertNotNull(trace.getOnResumeTime());
+    fakeExecutorService.runAll();
+    // There should be a trace sent since the delay between the main thread and onActivityCreated
+    // is limited.
+    verify(transportManager, times(1))
+        .log(
+            traceArgumentCaptor.capture(),
+            ArgumentMatchers.nullable(ApplicationProcessState.class));
+  }
+
+  @Test
+  public void testStartFromBackground_moreThan100ms() {
+    FakeScheduledExecutorService fakeExecutorService = new FakeScheduledExecutorService();
+    Timer fakeTimer = spy(new Timer(currentTime));
+    AppStartTrace trace =
+        new AppStartTrace(transportManager, clock, configResolver, fakeExecutorService);
+    trace.registerActivityLifecycleCallbacks(appContext);
+    trace.setMainThreadRunnableTime(fakeTimer);
+
+    when(fakeTimer.getDurationMicros()).thenReturn(TimeUnit.MILLISECONDS.toMicros(100) + 1);
     trace.onActivityCreated(activity1, bundle);
     Assert.assertNull(trace.getOnCreateTime());
     ++currentTime;
@@ -252,6 +284,7 @@ public void testStartFromBackground() {
     trace.onActivityResumed(activity1);
     Assert.assertNull(trace.getOnResumeTime());
     // There should be no trace sent.
+    fakeExecutorService.runAll();
     verify(transportManager, times(0))
         .log(
             traceArgumentCaptor.capture(),