[7/n] Create LetterboxController abstraction

Creates the LetterboxController abstraction to allow different
operations on LetterboxObserver callbacks.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 377857898
Test: atest WMShellUnitTests:LetterboxTransitionObserverTest

Change-Id: I8878901fb49c35fd6703956ffebdb8359d768707
7 files changed