* 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
e88a387b26