conference: identify mixed videos per streamId

Videos in the videoMixer_ were not easy to identify, moreover,
"sinkId" in the participants informations was the concatenation
of the confId and a URI, which is a problem if several devices
from the same account was present in the conference. Finally,
the active stream logic was dirty, with two different variables
used to identify the active stream in the mixer.

This patch introduces a streamId which is (callId_type_idx, e.g.
ca111412_video_0, ca111412_audio_2), so every video shown in the
conference are identified via a unique ID.
Active stream in the video mixer is identified by this ID, not
by the callId or pointer.

This should not change any behaviour, but prepare for multistream.

jami-project#1429

Change-Id: I250dd31ad1ea92ed1fd1e94bec2f5abd311d2128