Simulcast, Screen sharing & Various improvements (#4)

* Respect `RTCIceTransportPolicy` enum and organize

* Simplify syntax where possible etc.

* Combine `VideoPreset` and `VideoPresets`

* Default values for `ConnectOptions`

* Build URI instead of String manipulation

* Slight modifications to Exception

* `LiveKitTheme` for example

* `VideoEncoding` class

* Organize imports

* First simulcast implementation

* Remove unnecessary try-catches

* Update Android settings

* Remember uri and token

* example improvements

* `fit` parameter for VideoTrackRenderer

* Simulcast option for example

* Pass `defaultPublishOptions`

* Show only `VideoQuality`

* Pass tests

* Better buildUri logic

* Named parameter to positional

* Explicit imports

* `VideoParameter` instead of `VideoPreset`

* Use `mediaTrack.getSettings` when possible

* Safer dispose logic

* Safer `PCTransport`

Update transport.dart

* Synchronized events for `SignalClient`

* Use logger instead of print

* Make example compile for iOS

* First screen share implementation

* Make example work with screen share

* Example improvement

* Code optimization

* Don't depend on web_socket_channel

* Fix: Unpublish track bug

* Show participant mute state & identity

* Update protos

* Remote mute/unmute

* iOS Background mode

* Separate `createCameraTrack` and `createScreenTrack`

* Clean up

* PB fix

* format

* Fix analyzer warning

* Android clean up

* Update README.md

* Clean up

lib/src/track/video_track.dart

@@ -1,29 +1,37 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_webrtc/flutter_webrtc.dart';
 
-import '../proto/livekit_models.pb.dart';
+import '../proto/livekit_models.pb.dart' as lk_models;
 import 'track.dart';
 
 /// A video track will notify when its mediaTrack has changed.
 class VideoTrack extends Track with ChangeNotifier {
-  MediaStream? _mediaStream;
+  MediaStream _mediaStream;
 
-  VideoTrack(String name, MediaStreamTrack mediaTrack, this._mediaStream)
-      : super(TrackType.VIDEO, name, mediaTrack);
+  VideoTrack(
+    String name,
+    MediaStreamTrack mediaTrack,
+    this._mediaStream,
+  ) : super(
+          lk_models.TrackType.VIDEO,
+          name,
+          mediaTrack,
+        );
 
-  MediaStream? get mediaStream => _mediaStream;
+  MediaStream get mediaStream => _mediaStream;
 
   /// internal use
   /// {@nodoc}
-  set mediaStream(MediaStream? stream) {
+  void setMediaStream(MediaStream stream) {
     _mediaStream = stream;
     notifyListeners();
   }
 
   @override
-  stop() {
-    super.stop();
-    _mediaStream?.dispose();
-    _mediaStream = null;
+  Future<void> stop() async {
+    await super.stop();
+    await _mediaStream.dispose();
+    // _mediaStream = null;
   }
 }