feature: switch between front and back cameras

lib/src/track/video_track.dart

@@ -1,18 +1,27 @@
+import 'package:flutter/material.dart';
 import 'package:flutter_webrtc/flutter_webrtc.dart';
 
 import '../proto/livekit_models.pb.dart';
 import 'track.dart';
 
-class VideoTrack extends Track {
-  MediaStream? mediaStream;
+/// A video track will notify when its mediaTrack has changed.
+class VideoTrack extends Track with ChangeNotifier {
+  MediaStream? _mediaStream;
 
-  VideoTrack(String name, MediaStreamTrack mediaTrack, this.mediaStream)
+  VideoTrack(String name, MediaStreamTrack mediaTrack, this._mediaStream)
       : super(TrackType.VIDEO, name, mediaTrack);
 
+  MediaStream? get mediaStream => _mediaStream;
+
+  set mediaStream(MediaStream? stream) {
+    _mediaStream = stream;
+    notifyListeners();
+  }
+
   @override
   stop() {
     super.stop();
-    mediaStream?.dispose();
-    mediaStream = null;
+    _mediaStream?.dispose();
+    _mediaStream = null;
   }
 }