This PR introduces a `bool useSystemColors` parameter to the `ThemeData`
constructor.
The goal from this PR is to enable users to easily create high contrast
themes that are based on system colors for their `MaterialApp`:
```dart
MaterialApp(
theme: ThemeData.light(),
darkTheme: ThemeData.dark(),
highContrastTheme: ThemeData(useSystemColors: true, ...),
highContrastDarkTheme: ThemeData(useSystemColors: true, ...),
)
```
The `MaterialApp` widget will automatically pick the correct one of the
4 themes based on system settings (light/dark mode, high contrast
enabled/disabled).
Depends on https://github.com/flutter/flutter/pull/164933
Closes https://github.com/flutter/flutter/issues/118853
d5751be500