While adding transceivers from SetRemoteDescription,
the filtered codecs could filter out the primart codec
and leave the RTX codec in. Generating an answer with
that fails `SetRemoteDescription` on remote peer due
to an unrecognisable codec. Fix it by filtering out
RTX is primary is not there.
#### Description
Before this, any interceptor that tries to read FEC payload type from
`info *interceptor.StreamInfo`, always gets 0. This PR implements the
logic to find FEC payload type.
Also, MimeType consts are moved from mediaengine.go to mimetype.go,
since mediaengine.go is not included in the WASM build.
