Improve user notifications

[KyleFin]

Toasts suggesting page refresh and screenshot too soon in initialImpl are helpful.

Consider adding more messaging for things like:

• Attempted to search for text in current screen (every time so user knows we attempted and they can understand frequency of attempts? Every N times to remind user the service is running and they may want to turn it off?)
• After failed to find match N times? (Remind user the service is on when they aren't actively using it. I want to investigate performance of leaving the service running constantly. Maybe it's fine.)
• Scrolled
• Highlight text that was matched?
• Other failures?

[KyleFin]

As @alexschneider mentioned in #9 (comment), it would be great to hide the Live Caption view. I don't know if an AccessibilityService has permission to hide views from another app, but it would be great.

An alternative is to recommend users make the Live Caption background (and text) as transparent as possible. Quick experimentation on my Pixel phone suggests setting:

• Text color white, opacity 25%
• Background color none
• Caption window color black, opacity 25%
  in Caption size and style settings makes the Live Caption background transparent and the text a translucent light gray. This seems to not distract much from the transcript and useful to see captions are working and to know where to tap/drag to move or dismiss them.

To begin with, these suggestions could be documented. It may be helpful to make a setting to quickly (or automatically) set caption colors while Live Scroll Transcript is enabled and restore them when it's disabled.