* Gesture recognizer cleanup

1) Make OneSequenceGestureRecognizer.addAllowedPointer()
   call startTrackingPointer(), and change subclasses to
   call super.addAllowedPointer() in place of manually
   calling startTrackingPointer().
2) Fix addAllowedPointer overrides to take PointerDownEvent
   where some were taking PointerEvent.
3) Add API documentation to OneSequenceGestureRecognizer
4) Make the following fields in OneSequenceGestureRecognizer
   private with public getters instead of publicly writable:
   `state`, `primaryPointer`, and `initialPosition`.
5) Clean up gesture recognizer state in
   OneSequenceGestureRecognizer.didStopTrackingLastPointer.

Fixes #81883