Don't back up wallpapers that we've been told not to

In addition, now that the full uncropped wallpaper image is being
backed up, we now handle that via the full-data backup path instead
of key/value.  Restore still knows about legacy data that gets
delivered via the older key/value mechanism.

This change also has the effect of removing the size limitations
around wallpaper restore acceptance.  Any size source imagery is
valid, as crop & scale are rerun in a device-appropriate way
after the restore.

Bug 25453848
Bug 25727875

Change-Id: Idc64a2eaab97a8ecc9d2b8ca5dc011f29cab324d