[83/n] Fix Enter to Desktop and improve empty letterbox bounds case

The check on eligible task needs to be used at this stage.

When the letterboxBounds are empty we must assume the activity
is not letterboxed.

Flag: EXEMPT Refactoring
Bug: 430486865
Test: atest WMShellUnitTests:LetterboxLifecycleControllerImplTest

Change-Id: I85c6d455b6cd06525db7f77449e1993f77363fd4