conference/auto-answer: fix request media change

In conference and auto-answer mode, if the remote peer requests
a media change, the remote media was used to configure the local
media. This is bad, since if the remote muted its audio, it will
also mute the local audio.
Now, the existing media will not be modified, and the new media
will be automatically added if any.

Gitlab: #688

Change-Id: Id5388ed916eaa5755202b4b5b5fad118f0dc9c1e