STL: Reset PriorityNestedScrollConnection on first pointer down

Context: ag/28322124 moved the DraggableHandler from
NestedScrollToSceneNode to SwipeToSceneNode.

During this move, we overlooked the possibility of a scrollable
component unexpectedly disappearing from the composition, resulting in
the loss of some events.
The most important is "pointer up", which triggers the "onPreFling()"
call, serving as a signal to reset the tracking of the scroll consumed
by the SceneTransitionLayout children.

This could cause a flickering effect because specific
transitions might not be terminated at the appropriate time.

In this CL, we introduce a new approach to reset the tracking.
When the first pointer is detected on the screen, the tracker of the
scroll already consumed is reset.

Test: atest NestedScrollToSceneTest
Bug: 336710600
Flag: com.android.systemui.scene_container
Change-Id: If91d86531839cf16778b7818acfe8c9192ef0fa0