Remove wallpaper activity when moving to fullscreen/splitscreen

- Add remove wallpaper activity hierarchy ops when moving to
  fullscreen/splitscreen the last visible non-minimized task from desktop windowing.
- Removing wallpaper activity fixes the problem with navigating back to
  empty desktop (wallpaper activity without desktop windows).

Bug: 352481893
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Flag: com.android.window.flags.enable_desktop_windowing_wallpaper_activity
Change-Id: I80cb8e05ba40673293ea093e6e0e95368670a17f