Modify SDP to enable Opus FEC

A previous commit of enabling Opus FEC was based on the assumption that a peerconnection constraint can make WebRTC add "useinbandfec=1" in SDP. This was not achieved. This change lets AppRTC do the job.
this is duplicated with the addStereo function. Suggest adding a new findFmtpLine(sdp, codec) function that takes a string like "opus/48000" and returns the index of the fmtp line.