Check if the wallpaper service exists before use.

Add a null pointer check before calling the methods of
WallpaperConnection.mService.
Under normal circumstances, if the service is disconnected, we only
need to wait until receive WallpaperConnection#onServiceConnected.

Fix: 121181553
Test: Manually test on multi display environments by kill
wallpaper services, including SystemUI and selected wallpapers.

Change-Id: Ic679ba5393f658f3071b017551ee845035d0dfd6