* Use mapped SDP answer when using single peer connection mode.
In single peer connection mode, the client does not get the
TR_ ids for tracks. That is because the transceiver is created with a
receiver when the offer is generated and a random id assigned. Once the
id is assigned, it is not updated. So, when client gets `onTrack`, the
id of the associated MediaStreamTrack is not the TR_ variety.
So, send the mapping of mid -> trackID in signalling when sending
answer. And used that on client side to find LK track ID for given mid.
Side note: Probably worth doing it for dual peer connection also when
subscriber sends offer so that we do not have to deduce it on client
side using some rules.
* generated protobuf
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