[41/n] TaskVanishedListener for Letterbox surfaces removal

A TaskVanishedListener implementation that delegates to the
LetterboxController the removal of all the letterbox surfaces for
a closing Task.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 416222124
Test: atest WMShellUnitTests:LetterboxCleanupAdapterTest

Change-Id: I9c74e5295b58e2c6b78b47f46b2216278c609e21