STL dispatches events using the Nested Scroll system

Now, the MultiPointerDraggable in SceneTransitionLayout is part of the
Nested Scroll system.

Drag events are exposed as scrolls, and ancestors can intercept them
during the preScroll and postScroll phases.
Stop and cancel events are exposed as fling events, and they can be
intercepted in the onPreFling and onPostFling phases.

Test: atest MultiPointerDraggableTest
Bug: 336710600
Flag: com.android.systemui.scene_container
Change-Id: I5df016c3e9249112f7586c94857874dcac693029