[13/n] Implement MixedLetterboxController

Implement the MixedLetterboxController as coordinator
of other LetterboxController implementation like the ones
responsible for the letterbx surfaces lifecycle.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 377875146
Test: atest WMShellUnitTests:LetterboxUtilsTest
Test: atest WMShellUnitTests:MixedLetterboxControllerTest
Test: atest WMShellUnitTests:LetterboxTransitionObserverTest

Change-Id: I6359f963dba7d7102cdbfa91baacf38d83700e23