[4/n] Implement LetterboxObservable Tests

Implement a Type Safe Builder for testing TransitionObservable and
use it for testing LetterboxObservable.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 370997904
Test: atest WMShellUnitTests:LetterboxTransitionObserverTest

Change-Id: Ib36170c1ca621f37dd717686aa41b4256d417f98
2 files changed