create media list view controller to handles media list view

packages/stream_chat_flutter/lib/src/media_list_view_controller.dart

@@ -0,0 +1,16 @@
+import 'package:flutter/material.dart';
+
+/// Controller for MediaListView Widget
+class MediaListViewController extends ChangeNotifier {
+  var _shouldUpdateMedia = false;
+
+  /// Getter that knows if the media should be updated.
+  bool get shouldUpdateMedia => _shouldUpdateMedia;
+
+  /// Method that update shouldUpdateMedia and notify all listeners
+  /// about this update.
+  void updateMedia({required bool newValue}) {
+    _shouldUpdateMedia = newValue;
+    notifyListeners();
+  }
+}