fix(codec-selection) Calc preferred codec after media session is established.

Fixes an issue on mobile where the videos of users joining after the mobile endpoint are not being rendered.

modules/RTC/CodecSelection.js

             + `disabled=${this.jvbDisabledCodec} P2P: preferred=${this.p2pPreferredCodec},`
             + `disabled=${this.p2pDisabledCodec}`);
 
+        this.conference.on(
+            JitsiConferenceEvents._MEDIA_SESSION_STARTED,
+            session => this._selectPreferredCodec(session));
         this.conference.on(
             JitsiConferenceEvents.USER_JOINED,
             () => this._selectPreferredCodec());