[45/n] Integrate LetterboxLifecycleEventFactory

Integrate LetterboxLifecycleEventFactory in LetterboxTransitionObserver
to detect what LetterboxLifecycleEvent to generate from a Change.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 409043134
Test: atest WMShellUnitTests:MigrationLetterboxTransitionObserverTest
Test: atest WMShellUnitTests:DelegateLetterboxTransitionObserverTest

Change-Id: Ie8c2037ce8c8520c9bbb1a8df0bf0575806e8e68