Desks: Remove desk when last task is removed

In touch-first displays, remove the desk (so that it does not show in
Overview) when the last task in the desk is closed or moved out of
desktop.

Flag: com.android.window.flags.remove_desk_on_last_task_removal
Bug: 414845396
Test: atest WMShellUnitTests
Test: manual - open fullscreen app, then another fullscreen app and drag
it to desktop, then close it and verify there is no empty desk in
overview

Change-Id: I141da2454f21db4de0fe24ba11cdd2da3da53be7