audio_sender: mute state must be kept when restarting medias

When settuping the AudioSender, the audio can be started with
muteState_ = true. This avoid the case where the peer needs to
renegotiate the medias (like changing their camera resolution,
stopping a stream, etc) the audio will be restarted forgetting
the mute state.

Change-Id: Ic33e297809a72f4c7f85c1cdd354f99d953618cc
GitLab: #327