GsmConsoleKit: Properly initialize upower client

Move UPower client initialization from gsm_consolekit_on_name_owner_changed()
(which is not actually called for UPower usually) into
gsm_consolekit_ensure_ck_connection().

This makes the UPower calls actually work, otherwise they fail on trying to
pass a NULL client.

https://bugzilla.gnome.org/show_bug.cgi?id=688229