fix: update call participant video state when changed [WPB-2494]

[PatrykBuniX]

WPB-2494 [Web] Video stream not displayed anymore when leaving and rejoining a call while video is received

Fixes the issue where video/screenshare of other call participants is not visible after rejoining the call.

We initialise call participants with video state 0 (STOPPED). Previously we were just listening to videoStateChanged (vstateh) event to update videoState of participants in webapp client. When rejoining the call this callback is not really fired since video state of the other users does not really change (we're the one who's joining the call), so webapp was never informed again that some participants are sharing video/screen.

The solution is to update video state of each participant whenever participant list changes - wCall.setParticipantChangedHandler(wUser, this.handleCallParticipantChanges). This callback gets fired if any state of call participant changes - whether it is audio establishment (aestab), video state (vrecv) or muted. When joining a call user is informed of what is the current state of call participants. For more details see https://wearezeta.atlassian.net/wiki/spaces/ENGINEERIN/pages/616857724/Use+case+observe+call+participants

[codecov]

Codecov Report

Merging #15421 (986da69) into dev (5218339) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##              dev   #15421   +/-   ##
=======================================
  Coverage   43.34%   43.34%           
=======================================
  Files         647      647           
  Lines       21825    21826    +1     
  Branches     5016     5016           
=======================================
+ Hits         9459     9460    +1     
  Misses      11148    11148           
  Partials     1218     1218