stasis: Only place stasis created and dialed channels into dial bridge.

stasis: Only place stasis created and dialed channels into dial bridge.

The dial bridge is meant to hold channels which have been created
and dialed in stasis. It handles the frames coming from them and raises
the appropriate events.

It was possible for the code to mistakenly place calls which came
from the dialplan into the dial bridge if they were not in an
answered state. These channels are not outgoing channels and
should not be placed into the dial bridge.

The code now checks to ensure that only stasis created channels are
placed into the dial bridge by checking that a PBX does not exist
on the channel.

ASTERISK-27756

Change-Id: Ideee69ff06c9a0b31f7ed61165f5c055f51d21b6