MediaFactory: Clarify references to GabbleCallChannel
MediaFactory: Clarify references to GabbleCallChannel

When I was debugging the leak fixed by the previous patch, I found it
hard to track the ref(s) owned by the media factory to the call channel.
This patch just makes the passing of references more explicit, rather
than passing a single reference around.

Ziemin/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble