Release lock before color extraction on dim changes

notifyWallpaperColorsChanged should always be called outside the lock
since it may trigger heavy color computations.

This is necessary in order to remove the old code paths of
clearWallpaperLocked without causing boot time regressions.

Flag: lockscreen live wallpaper
Bug: 273443374
Test: atest WallpaperManagerTest
Change-Id: Ib2b6dde8b65058874d2a4c014fa7de47320e6457