Keep selected wallpaper on configuration change.

Keeps the currently selected wallpaper when a configuration (for example,
device rotation) occurs. This change also effects the logic when the thumbnail
gets displayed after loading the image from an external URI, as this must be
now handled independently of which wallpaper is selected.

Bug: 11007874
Change-Id: Id676c378aec38d0b21e5d9150846f33aff415cad