Remove synchronous wallpaper callbacks

- Synchronous wallpaper offsets were removed via a flag
  (wallpaper_offset_async) which makes offset callbacks always
  asynchronous.

- Synchronous wallpaper commands never worked because they
  have had the plumbing on the client side but the server side has
  had an open TODO for more than 5 years to implement waiting.

Fix: 409612162
Test: atest WallpaperControllerTest
Flag: EXEMPT cleanup
Change-Id: I3c47e506abca4afaefa11077112f8de1ecfa0b57