[50/n] Rename leash to taskLeash to use in Letterbox Surface creation

Makes more explicit the fact that the leash in this case is the
leash of the Task and it's used as parent surface for the
letterbox surfaces.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 375339716
Test: atest WMShellUnitTests:LetterboxLifecycleControllerImplTest
Test: atest WMShellUnitTests:LetterboxLifecycleEventTest
Test: atest WMShellUnitTests:TaskInfoLetterboxLifecycleEventFactoryTest

Change-Id: I47cfc9b55710d47ea395b3c15cab1c75b562228d