Refactor remote codec update handling.
Refactor remote codec update handling.

Previously, if two updates were received, and the second was invalid,
the first would been applied to the cache before rejecting the
description-info. This would mean that any subsequent valid
description-infos would cause SetRemoteCodecs to be emitted including
the erroneously accepted change from the invalid stanza.

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