Fix wallpaper NPE caused by user switching before IWallpaperService connection

- car side is doing quick user switching during boot-up and that leads into
  wallpaper user switching before the service connection.
  user 0 start, user 0 unlock, user switch to user 10, boot complete, user 10 unlock

Bug: 129569939
Test: Try user switching flow in car env and confirm no crash
Change-Id: I764f68377ddf8b581f8386a236aa5c24a96368dd