Currently, the media attribute in the SDP is always set to
'sendrecv' regardless of the mute state of the media.
In this patch, media direction will be set according to mute
state of the media.
Note that this only applies if the mute/unmute requires media
renegotiation (SIP re-invite with new SDP session). Currently,
this only the case for video media. For audio, mute/unmute is
done locally without SIP re-invite.

References:
RFC-3264
RFC-4317 (non-normative)

Gitlab: #645

Change-Id: I604331255bd25dfe732e192039a673a0980105fa