The value that value or attributedValue will become after a
SemanticsAction.decrease action has been performed on this widget.
If a value is provided, onDecrease must also be set and there must
either be an ambient Directionality or an explicit textDirection
must be provided.
Callers must not provide both decreasedValue and
attributedDecreasedValue, One or both must be null.
final String? decreasedValue;