Release camera/mic when muted, restartTrack when unmuted (#39)

* re-structure tracks

* publication.track is now a LocalTrack or RemoteTrack

* restartTrack when unmute

* stop/start on restartTrack

* move to mute/unmute

* format

* remove comment

lib/src/track/remote_track_publication.dart

@@ -12,6 +12,7 @@ import '../participant/remote_participant.dart';
 import '../proto/livekit_models.pb.dart' as lk_models;
 import '../proto/livekit_rtc.pb.dart' as lk_rtc;
 import '../utils.dart';
+import 'remote_track.dart';
 import 'track.dart';
 import 'track_publication.dart';
 
@@ -31,6 +32,9 @@ class RendererVisibility {
 /// Represents a track publication from a RemoteParticipant. Provides methods to
 /// control if we should subscribe to the track, and its quality (for video).
 class RemoteTrackPublication extends TrackPublication {
+  @override
+  covariant RemoteTrack? track;
+
   final RemoteParticipant _participant;
   bool _enabled = true;
   lk_rtc.VideoQuality _videoQuality = lk_rtc.VideoQuality.HIGH;