Don't try to sync wallpapers

Wallpapers currently don't support sync, so it doesn't make sense
to try and sync with them -- otherwise we get timeouts waiting for
a signal that will never be fired

Bug: 238328090
Test: atest NexusLauncherTests (and observe logs for no related timeouts)
Change-Id: I83a3489c0790975eb80a7030cf3d1129d4746513