wallpapers/potd: fix not updating wallpaper in lock screen

Skip updating wallpapers in client's ctor. Call `setUpdateOverMeteredConnection`
or `updateSource` in `PotdBackend::registerClient()` , to ensure only
one loading process will be run at the same time.

BUG: 457361
FIXED-IN: 5.26


(cherry picked from commit 20c0881e)