renderer: implement PlaybackStorageMedium variable

PlaybackStorageMedium should be "NETWORK" when AVTransportURI
refers to a resource received from the UPnP network. It should be
NONE if AVTransportURI is not set.

Also fix the LastChange value of PossiblePlaybackStorageMedium.

https://bugzilla.gnome.org/show_bug.cgi?id=712517