This PR mainly fixes several memory leaks in the `DateRangePickerDialog`.

### Description
- Fixes https://github.com/flutter/flutter/issues/136033 by:
    1) adding a disposal of several `RestorableValue`;
    2) creating a separate `_DayItem` stateful widget that creates/updates/disposes internal `MaterialStatesController`.
- Marks https://github.com/flutter/flutter/issues/136036.

### Tests
- Updates `test/material/date_picker_theme_test.dart` to use `testWidgetsWithLeakTracking`;
- Updates `test/material/date_range_picker_test.dart` to use `testWidgetsWithLeakTracking`.