Dismissing with correct velocity when flinging and not detaching.

If the user flings a notification without detaching it, the detach
direction should be 0. In this case, the SwipeHelper should use an
unmodified velocity to run the dismiss animation. At the same time, the
internal direction estimator of MagneticRowManagerImpl should reset the
direction to 0 when called to reset()

Test: manual. Verified notifications dismiss in the correct direction
  when flinging right and left without magnetic detachtments.
Test: added Unit test
Flag: com.android.systemui.magnetic_notification_swipes
Bug: 416012576
Change-Id: Ibac2d86499340e013c42dbf897e5eb72637c1041