This change breaks the existing ScrollStateDetector implementation
into two. It extracts an interface that users can potentially
implement to provide their own ScrollStateDetector. It also creates
an additional DefaultScrollStateDetector class that now contains the
default ScrollStateDetector implementation.

This change adds onInterceptTouchEvent support to ScrollStateDetector
and the relevant hooks in LithoScrollView and HorizontalScrollSpec.
The implementation in the DefaultScrollStateDetector will be a no-op,
because in that class all detection is done through onTouchEvent.
Finally it also adds documentation in the ScrollStateDetector
interface.

This change addds @nullable for ScrollStateDetector in both the
HorizontalScrollSpec and VerticalScrollSpec. It also changes the
method names in DefaultScrollStateDetectorTest to conform to the
Litho standard.

…tectors