app_confbridge: Move participant info code to confbridge_manager.

With the participant info code in app_confbridge, we were still
in the process of adding the channel to the bridge when trying to send
an in-dialog MESSAGE.  This caused 2 threads to grab the channel
blocking flag at the same time.  To mitigate this, the participant
info code was moved to confbridge_manager so it runs after all
channel/bridge actions have finished.

Change-Id: I228806ac153074f45e0b35d5236166e92e132abd