BugFix: Set default UID when reading in the groups

With v0.8 I have found an issue when calling "play_from_queue" that the UID is "None" - it does a call to the "uid" property, that then checks self._uid - if that is not set yet it calls: _parse_zone_group_state()

But this method currently does not actually set the self_uid that this then returned fro the "uid" property.