Multi-stream: Create and update media of an outgoing call

Add APIs to set a list of desired media to included when making
outgoing calls.
Also add APIs to update the media of an ongoing call. The update
can be used for example to "mute/un-mute" a media, or to add a new
media (add video to an audio-only call).

Gitlab: #380

Change-Id: Iab32a72547f3a0a2328408aadc6a7656c971a7b0