Drag events can only start if there is at least one pointer down.

In this CL, there needs to be at least one pointer down. That's because,
 in a later CL, the initial position will be calculated from the instant
  the pointer touches the screen and it'll be nullified when there are
  no more pointers on the screen.

Test: atest MultiPointerDraggableTest
Bug: 330200163
Flag: com.android.systemui.scene_container
Change-Id: I58fd5cc2fefeff1284b492c1313dfb64bc12044d