[1/n] Remove top transparent fullscreen task on new task opening

When a new task is opening or getting brought to front, remove the top
transparent fullscreen task in the current wct to forcefully close it.
The closing of this top transparent task will be handle and animated by
the SystemModalTransitionHandler.

Flag: com.android.window.flags.force_close_top_transparent_fullscreen_task
Bug: 395041610
Test: atest WMShellUnitTests:DesktopTasksTransitionObserverTest,
      atest WMShellUnitTests:DesktopTasksControllerTest,
      atest WMShellUnitTests:DesktopRepositoryTest,
      atest WMShellUnitTests:DesktopMixedTransitionHandlerTest
Change-Id: I792901711ebb57a63f6d1f4fdf091f8d0c8a5a00