rfkill: clear id after unowning bus name

Or we'll try to unown the same bus id again next time
gsd_rfkill_manager_stop() is called.

https://bugzilla.gnome.org/show_bug.cgi?id=724573
updates: don't use changed signal on UpClient

This was removed recently from upower; connect to the notification of
the property we're interested in instead.

https://bugzilla.gnome.org/show_bug.cgi?id=724573