SIP call: re-invite without ice re-negotiation

When muting/un-muting the video, a re-invite is performed
leading to a full media renegotiation and restart,  including
ICE session if used.
With these changes, the mute/unmute video will still require
a re-invite (a new SDP to indicate the new media directions), but
the ICE session is re-used and only the video is stopped/started
accordingly.
The behavior improves the UX by avoiding unnecessary audio disruptions
and is more compliant with SIP/ICE specs (see RFC-5245 section 9.1.1.1
for example)

Gitlab: #671

Change-Id: I13caf9a965af1d76e922fe5f6b86d5332b3296d6