[Flexiglass] Don't expand the shade by dragging on a notification over lockscreen

The motion to expand a notification over the lockscreen changes with
flexiglass. The drag will only expand the notification first (as opposed
to control the shade expansion), and if the user has dragged far enough,
it will expand the shade when the gesture is finalized.

This CL achieves this by NOT setting NSSL#mIsBeingDragged to true, from
a gesture that is expanding a notification over the lockscreen.

Bug: 359957196
Test: Drag down on a Notification on the LS -> verify that
       - the shade is NOT expanding during the gesture
       - touches during the gesture are not dispatched to the scene framework
       - if dragged far enough, finishing the gesture opens the shade
       - if not dragged far enough, finishing the gesture collapses the
	 notification back to its original lockscreen height
Flag: com.android.systemui.scene_container

Change-Id: Ifc669f4ad1318d758ef7e3d9899c6a8a697ae7fa