[10/n] Handle recents in Letterbox surfaces lifecycle

We don't need to remove the letterbox surfaces for activities
entering recents.

Flag: com.android.window.flags.app_compat_refactoring
Fix: 379257107
Test: atest WMShellUnitTests:LetterboxTransitionObserverTest

Change-Id: I2b53d544daab935a90f398c2527cba76b630a37f