ImageWallpaper: forget wallpaper in background

Forgetting the wallpaper requires holding the wallpaper
manager lock - which is also held when the bitmap is
read from disk. To prevent blocking the main thread,
we need to offload the forget operation to the background
too.

Change-Id: Ic20c4f5fd86b788efd5b77a417108383dda343b9
Fixes: 28769940
Test: manually change wallpaper, rotate screen