Update `flutter_localizations/test` to stop using `TestWindow`

There are currently 12 references in `flutter_localizations` that make use of `TestWindow` properties, these should be updated to use `TestFlutterView` and `TestPlatformDispatcher` as appropriate for the use cases.

* `date_picker_test.dart` has 4 references
* `time_picker_test.dart` has 8 references