Please make AnimationController in ExpansionIndicatorState public

[Chris1234567899]

Hi, thanks for the great package. A minor thing though: For simplifying the process of creating a custom ExpansionIndicator (extending the abstract ExpansionIndicatorState class), it would be nice to get public access to its AnimationController.

Right now it is private: late final AnimationController _controller = AnimationController( duration: animationDuration, vsync: this, );

Could you make it public by removing the underscore pls?