Prevent restart of audio media stream if media didn't change

[MayamaTakeshi]

Currently, whenever on_media_update is called, we restart the media stream with the new local and remote SDP media description.
This causes a small click sound in the audio.
However, if the on_media_update happens due to simple REINVITE for session refresh without changing the media characteristics and send/recv state, then we can prevent this sound from happening.
Let's check pjsua code to see how this is done.