Don't stomp live wallpapers when tidying up imagery

Tidying up the wallpaper imagery bookkeeping mustn't accidentally
overwrite any user-selected live wallpaper usage.  Only do the full
reset when it's needful *and* we expect to need it.

Bug 27537903

Change-Id: Iaacc750a24ef36ee4e4cc436b62055b51b49b235