STL MultiPointerDraggable compute direction using the previousPosition

If the user drags in the opposite direction, the delta becomes zero
because we return to the original point.
Therefore, we should use the previous event to calculate the direction.

Test: atest MultiPointerDraggableTest
Bug: 379403509
Flag: com.android.systemui.scene_container
Change-Id: I5065f65ed8995b95245c7b744fb4b5d019f416fd