Using correct start velocity for re-attaching animation.

When we re-attach a notification, we should set the spring initial
velocity so that it goes in the opposite direction in which the
detachment happened. This bug was causing the spring to start in the
wrong direction, resulting in a visible jump-cut when fidgeting with the
magnetic attachment.

Test: manual. Verified re-attaching is smooth
Flag: com.android.systemui.magnetic_notification_swipes
Bug: 416415702
Change-Id: I6d51264fbb52ef89e147fd39dd22b226457e7e08