Detach engines before destroying WallpaperService

WallpaperService contained a race condition where the service might be
destroyed before all its engines had been detached. For subclass
ImageWallpaper this would leak the engine's BroadcastListener, which in
turn would trigger a system crash (as ImageWallpaper runs as part of
the system).

Change-Id: Ia900173a1d3070cbf6bcf769f68376c8c8cbc62a