Check if we have already an OlpcActivity for that channel
Check if we have already an OlpcActivity for that channel

Otherwise we were creating two activities for the same channel: one
when the channel is created and the other when Sugar calls AddActivity.

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