Rework the high contrast accessibility feature

[swift-kim]

Currently a boldText flag is being passed to the engine when the High Contrast option is set in the TV settings. A highContrast flag must used instead.

Contributes to flutter-tizen/flutter-tizen#151. Should be rebased upon #264.

The invertColors feature support will be added after this change.

Code example:

return MaterialApp(
  theme: ThemeData(colorScheme: const ColorScheme.light()),
  highContrastTheme:
      ThemeData(colorScheme: const ColorScheme.highContrastLight()),
  ...
);