Back up / restore lock wallpapers

If writing both system + lock wallpapers winds up hitting the
transport-defined quota, the next backup operation steps back
to storing only the system wallpaper.

Also makes sure to unbind full-backup target agents following
the backup operation.  In practice this usually doesn't matter
because the target gets killed following the operation, but
the wallpaper agent runs in the system process where this does
not happen, so was mistakenly being left in place and reused
for the next operation, failing to re-run the full create +
backup lifecycle.

Bug 28968107

Change-Id: I219c2ddd7e899a430ef4cf693b1259464c15eed5