Add audio only calls support

lib/models/call_member.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_webrtc/media_stream.dart';
 
 /// Single call member information
 ///
@@ -9,10 +10,14 @@ enum MemberStatus { JOINED, READY }
 class CallMember {
   final int userID;
   MemberStatus status;
+  MediaStream stream;
 
   CallMember({
     @required this.userID,
     this.status = MemberStatus.JOINED,
   })  : assert(userID != null),
         assert(status != null);
+
+  bool get hasVideoStream =>
+      stream != null && stream.getVideoTracks().length > 0;
 }