Preserve shared {system+lock} wallpaper when setting system only

We migrate the existing (shared) system wallpaper to be the lock-only
wallpaper when setting a new system-only wallpaper.  The end result
is that if you say "set the system wallpaper only" the lock wallpaper
won't change.

(The migration is via a rename of the underlying imagery files, not
via copying, because copying all that data would be silly.)

Bug 27599080

Change-Id: I03ecf23c943fe88af58d5ac26f05587a15e2d0a9