Multi-user - wallpaper service

- Allow each user to have their own wallpaper (live or static).
- Migrate old wallpaper on upgrade.
- Update SystemBackupAgent to backup/restore from primary user's
  new wallpaper directory.

Reduce dependency on Binder.getOrigCallingUser() by passing the
userId for bindService.

Change-Id: I19c8c3296d3d2efa7f28f951d4b84407489e2166