bridge: Don't try to match audio formats.

When bridging channels we were trying to match the audio
formats of both sides in combination with the configured
formats. While this is allowed in SDP in practice this
causes extra reinvites and problems. This change ensures
that audio streams use the formats of the first existing
active audio stream. It is only when other stream types
(like video) exist that this will result in re-negotiation
occurring for those streams only.

ASTERISK-28871

Change-Id: I22f5a3e7db29e00c165e74d05d10856f6086fe47