[53/n] Define LetterboxTaskListenerAdapter

Creates LetterboxTaskListenerAdapter as adapter which
is useful to store some TaskInfo properties to use when
the TaskInfo object is not present in Change.

This also creates the test utils for testing all the
implementation of TaskAppearedListener, TaskVanishedListener
and TaskInfoChangedListener.

Flag: com.android.window.flags.app_compat_refactoring
Bug: 309593314
Test: atest WMShellUnitTests:LetterboxTaskListenerAdapterTest

Change-Id: I04d4da5a19a2c0dbc693e9e7a7ec2ab7711759ee