[1/n] Implement alternative translucent heuristic

At the moment Shell understands if the top activity is
transparent using the existence of FLAG_TRANSLUCENT.
Here the TaskInfo#isTopActivityTransparent is used
instead.

This is important for the transition of entering Desktop
Windowing with a letterboxed activity.

Flag: EXEMPT small fix
Bug: 433747415
Test: atest WMShellUnitTests:LetterboxLifecycleEventTest

Change-Id: Id607ce0ec6caa4ed2d0e534a75f26a71fb5fa19e