Support detection of light and dark system colors (#164933)

Part of https://github.com/flutter/flutter/issues/118853

This PR is an enhancement to
https://github.com/flutter/flutter/pull/163335 to provide detection of
system colors for both light and dark mode. This is needed for the
construction of a
[`highContrastTheme`](https://api.flutter.dev/flutter/material/MaterialApp/highContrastTheme.html)
and
[`highContrastDarkTheme`](https://api.flutter.dev/flutter/material/MaterialApp/highContrastDarkTheme.html).