Code review comment for lp:~boiko/telephony-service/preserve_call_status

I think there might be a case where this logic fails.
Correct me if I am wrong, but consider the case where we are on a call and switch to another app, then while dialer-app is in background the remote party hangs up. When you bring the dialer-app to foreground again and register the observer, onCallChannelAvailable() wont be called as there are no active calls at the moment, so we will end up in an inconsistent state.

139 void CallManager::onCallChannelAvailable(Tp::CallChannelPtr channel)
140 {
141 + // if this is the first call after re-registering the observer, clear the data