Improvements to M3 demo app (#1647)

* improvements

* fix focus

* add comment

* add comment

* copy changes to root material_3_demo

* fix large breakpoint

* fix large breakpoint

* Create integration_test.dart

* refactor main.dart into home.dart and constants.dart

* add integration_test to pubspec

* copy to root material_3_demo

* remove removal of constraints

* address feedback

experimental/material_3_demo/integration_test/integration_test.dart

@@ -0,0 +1,15 @@
+// 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_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:material_3_demo/main.dart' as app;
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+
+  testWidgets('verify app can run', (tester) async {
+    app.main();
+  });
+}