GabbleMediaChannel: listen to GabblePresenceCache::unsure-period-ended
GabbleMediaChannel: listen to GabblePresenceCache::unsure-period-ended

Previously we hard-coded a wait of 5 seconds, but if the call was
started (say) 2 seconds after the presence cache was created, then a
3 second wait would be adequate. We might as well let the presence
cache tell us what it's thinking, rather than duplicating the timeout
logic.

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