Add SET_WALLPAPER permission to SystemUI

If an exception is thrown in updateWallpaperLocked, a call to
WallpaperManager.clear() is made.  This requires the SET_WALLPAPER
permission and will throw a SecurityException if that permission
is not granted to SystemUI.

Change-Id: I61811b65dcfe8b15f0e9030f7aa6d1dc09cea351