manually handle audio tracks for web

lib/src/track/_audio_api.dart

@@ -0,0 +1,9 @@
+import 'package:flutter_webrtc/flutter_webrtc.dart';
+
+void startAudio(String id, MediaStreamTrack stream) {
+  // do nothing
+}
+
+void stopAudio(String id) {
+  // do nothing
+}