[28/n] Hide letterbox surfaces when activity is not letterboxed

The visibility of the letterbox surfaces is already handled
in the related LetterboxController. The only case when the
letterbox surfaces need to be explicitly and completely hidden
is when the top activity is not letterboxed.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 384473893
Test: atest WMShellUnitTests:LetterboxTransitionObserverTest

Change-Id: Id3ab5e60c1a4aef4fb795ab600a38872128874f8