[generator] Mark protected types nested in sealed classes as private.

[jpobst]

In #569 we marked protected members in sealed classes as private to prevent csc warnings. However we did not cover nested types which also cause this warning.

sealed class Foo
{
  protected interface Bar
  {
  }

  protected class Bar2
  {
  }
}

warning CS0628: 'Foo.Bar': new protected member declared in sealed class
warning CS0628: 'Foo.Bar2': new protected member declared in sealed class

Mono.Android.dll added 28 new protected interfaces in sealed classes CalendarContract and ContactContract which caused new warnings is our builds.

obj\Release\monoandroid10\android-30\mcw\Android.Provider.CalendarContract.cs(614,40): warning CS0628: 'CalendarContract.IAttendeesColumns': new protected member declared in sealed class [C:\A\vs2019xam00000Z-1\_work\1\s\src\Mono.Android\Mono.Android.csproj]

This PR marks nested types as private if the parent class is sealed.