Mute audio: do not change media direction

Mute/un-mute audio is done only locally, i.e. without requesting
a media change (re-invite) as done for the video, thus the media
direction in the SDP must not change for the audio stream.

Gitlab: #688

Change-Id: I3775a29f6c566a159d5b9269b4d9462ab4e3c36f