manager: auto-answer if two daemons are trying to call each other

When two persons are trying to call each others, we can assume they
want to talk together. So, the daemon should automatically answer.
It avoids some weird case where they both see the outgoing call view,
but cannot answer to the incoming call.

There is several scenarios.
1. Both are trying to make a video call, in this case, the daemon
with the incoming call from the higher ring_id should answer and drop
its outgoing call resulting in a video call.
2. Both are trying to make an audio call, same behavior as scenario 1,
resulting in an audio call.
3. One device is trying to make a video call, the other an audio call,
audio call is prioritary here, and the daemon with an incoming audio only
call should answer and drop the outgoing video call.

Change-Id: Id1e639448243c19bf492f2d057496cab6d05de8c
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>