STL ignore mouse wheel

MouseWheel type events now have their own designated type. This type is
emitted even when there are no pointers present on the screen.

The NestedDraggableHandler can use this event to determine whether to
ignore the event.

A new PointerInfo.PointerDown is now emitted only when pointers are
detected on the screen.

Test: atest SwipeToSceneTest
Bug: 371984715
Flag: com.android.systemui.scene_container
Change-Id: I254256dd2c5175618cad6289a0ebf5b82617e15d