AAC audio type conflicts.

[ynheng]

find reason: the core-libwebrtc does not support AAC audio type，but OWT has the AAC type. I read the webrtc source, there is no aac audio type in it，but there is a AAC enum type in OWT.

When I create a PublishOptions object, and add only one AAC audio type for it, then I set the PublishOptions param for Conference.publish(), It will call-back function onFailure(OwtError)，errorCode 3000，errorMsg "SessionDescription is NULL." but the createOffer() is success, after I analyze the SDP data，finally I found it missing the audio type, there is no available audio type in it； then I remove the publish options param, the publish function runs well, read the SDP data again，it has all available types now.