[36/n] Implement LetterboxLifecycleController

We decouple the logic related to TransitionObserver to the one
related to the lifecycle of the letterbox surfaces in Shell.
This will make easier to integrate the same logic with other
entry points (e.g. Listeners)

Flag: com.android.window.flags.app_compat_refactoring
Bug: 409951586
Test: atest WMShellUnitTests:LetterboxLifecycleControllerImplTest
Test: atest WMShellUnitTests:LetterboxLifecycleEventTest

Change-Id: Id0bfef10b7f549f112f8266573c62413cee25c8b