[2/n] Clear appBounds on exit from Desktop transition

To allow activity to be letterboxed when existing from
Desktop Windowing, it's necessary to reset the appBounds
from empty to null.

Flag: com.android.window.flags.app_compat_refactoring_set_appbounds_to_null_when_empty
Bug: 438406944
Test: atest WMShellUnitTests:DesktopTasksControllerTest

Change-Id: Ib4673b0a859f43e061dbfe3a91dd5bb78107352c