Handle cancellation in NestedDraggableNode.trackDownPosition()

This CL ensures that trackDownPosition() correctly clears the map of
pointersDown when the pointerInput is disabled.

Bug: 378470603
Test: atest NestedDraggableTest
Flag: EXEMPT NestedDraggable is not used yet
Change-Id: Ie3cd89b7622cc7cfbe7b28b9f24a23f783293a18