Start wallpaper activity as system user.

This cl changes the behaviour of the wallpaper activity to:
- Run on user 0
- To moved to back instead of being removed upon user exiting desktop
  windowing.

Test: atest DesktopTasksController
Test: atest DesktopTasksTransitionObserver
Flag: com.android.window.flags.enable_desktop_wallpaper_activity_on_system_user
Bug: 385294350
Bug: 381396057
Change-Id: I7604c00befbe9ac11dce61a33d57eae1f1316056