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

material_3_demo/lib/constants.dart

@@ -0,0 +1,40 @@
+// 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';
+
+// NavigationRail shows if the screen width is greater or equal to
+// narrowScreenWidthThreshold; otherwise, NavigationBar is used for navigation.
+const double narrowScreenWidthThreshold = 450;
+
+const double mediumWidthBreakpoint = 1000;
+const double largeWidthBreakpoint = 1500;
+
+const double transitionLength = 500;
+
+enum ColorSeed {
+  baseColor('M3 Baseline', Color(0xff6750a4)),
+  indigo('Indigo', Colors.indigo),
+  blue('Blue', Colors.blue),
+  teal('Teal', Colors.teal),
+  green('Green', Colors.green),
+  yellow('Yellow', Colors.yellow),
+  orange('Orange', Colors.orange),
+  deepOrange('Deep Orange', Colors.deepOrange),
+  pink('Pink', Colors.pink);
+
+  const ColorSeed(this.label, this.color);
+  final String label;
+  final Color color;
+}
+
+enum ScreenSelected {
+  component(0),
+  color(1),
+  typography(2),
+  elevation(3);
+
+  const ScreenSelected(this.value);
+  final int value;
+}