Reuse existing temporary wallpaper tiles when selecting same photo.

Previously, if you selected a photo to set as the wallpaper multiple times, it
kept adding temporary wallpaper tiles that were all identical. Now it reuses the
same tile, updating the thumbnail and wallpaper if the Uri happens to point to a
new image (such as if a photo is renamed).

Change-Id: Ib05c9db4e1fd17226b503174f25add3368158b11