[68/n] Add isTranslucent to LetterboxLifecycleEvent

LetterboxLifecycleEvent might contain information about
transparency to be used as a criteria for the multiple
or single letterbox surfaces.

LetterboxLifecycleEventUtils has been created accordingly and
dependent tests improved.

Flag: EXEMPT Refactoring
Bug: 377875146
Test: atest WMShellUnitTests:TaskInfoLetterboxLifecycleEventFactoryTest
Test: atest WMShellUnitTests:ActivityLetterboxLifecycleEventFactoryTest
Test: atest WMShellUnitTests:LetterboxLifecycleEventTest
Test: atest WMShellUnitTests:LetterboxTaskListenerAdapterTest

Change-Id: I86e7f1beb398abf6893fd5dfa838e6c7b0898dd0