Don't synchronize inside getWallpaperColors()

Reading the current wallpaper's palette doesn't involve any state
inside the WallpaperManager instance, so there's no reason to
apply synchronization in the first place.  By doing so we were
inducing a great deal of lock contention, since the system side
of the operation can sometimes take a long time to execute.

Bug 63065764
Test: manual

Change-Id: I6f5f67a0819da23245a4ad5334f0942011467f7c