Added license headers for M3 demo app (#1328)

* Added header for all files

Co-authored-by: Qun Cheng <quncheng@google.com>

material_3_demo/lib/main.dart

@@ -1,3 +1,7 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'package:flutter/material.dart';
 
 import 'color_palettes_screen.dart';