Load wallpaper colors in a background thread.

In WallpaperManagerService, it takes some time to load wallpaper colors
from image wallpapers since bitmap decoding will be involved. This will
block the UI thread and lead to app launch latency. So here we are
making it in another thread to avoid this.

Bug: 133396959
Test: robotest, reboot phone and look at boot trace
Change-Id: Ibd1952a4bf10431ba4be4dd69634d64354670daa