Desktop windowing wallpaper

* Add translucent wallpaper activity to move Launcher in the correct
  state
* Handle back navigation to remove the wallpaper activity if the last
  freeform window is removed
* Handle window decoration close to remove wallpaper activity if the
  last window is closed

Bug: 309014605
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_wallpaper_activity DEVELOPMENT
Test: atest DesktopTasksControllerTest DesktopModeTaskRepositoryTest
Change-Id: I38271dc3903ec35d53b4607f034b27d8390ac839