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

example/lib/widgets/controls.dart

@@ -104,10 +104,9 @@ class _ControlsWidgetState extends State<ControlsWidget> {
       await FlutterBackground.initialize(androidConfig: androidConfig);
       await FlutterBackground.enableBackgroundExecution();
 
-      final screenTrack =
-          await LocalVideoTrack.createScreenTrack(); // Defaults to camera
+      final screenShareTrack = await LocalVideoTrack.createScreenShareTrack();
       await widget.room.localParticipant.publishVideoTrack(
-        screenTrack,
+        screenShareTrack,
       );
     } catch (e) {
       print('could not publish video: $e');