Improve color display in M3 Demo (#2438)

- Added the color scheme schematic from Material Theme Builder aligning
how colors are displayed here and in documentation across Android and
Figma.
- On desktop, individual colors can be copied to the clipboard
- View adapts from the existing single column view to the new schematic
at 500dp width.

### Before:
<img width="1727" alt="Screenshot 2024-09-12 at 2 40 15 PM"
src="https://github.com/user-attachments/assets/37423d79-174a-4691-b0e1-8f18c947550a">


### After:


![mobile](https://github.com/user-attachments/assets/43a7bfbd-6217-4d3f-a1d6-902c99260905)


![Screenshot_20240912_144150](https://github.com/user-attachments/assets/48df7ad4-44a1-4802-8dac-06006dfb6bb1)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

---------

Co-authored-by: Eric Windmill <ewindmill@google.com>

material_3_demo/test/color_screen_test.dart

@@ -6,6 +6,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:material_3_demo/color_palettes_screen.dart';
 import 'package:material_3_demo/main.dart';
+import 'package:material_3_demo/scheme.dart';
 
 import 'component_screen_test.dart';
 
@@ -69,6 +70,6 @@ void main() {
     ));
     expect(find.text('Light ColorScheme'), findsOneWidget);
     expect(find.text('Dark ColorScheme'), findsOneWidget);
-    expect(find.byType(ColorGroup, skipOffstage: false), findsNWidgets(18));
+    expect(find.byType(SchemePreview, skipOffstage: false), findsNWidgets(2));
   });
 }