Fix velocity in StaggeredWorkspaceAnimation.

* Velocity should match the direction of the spring values.
  (Swipe direction is upwards, but icons move downwards on screen).
* Remove additional conversion to pxPerS since getDimension already does that.

Bug: 160003774
Change-Id: I12912edb2354c4a30c538da6ca3789c46d82b94d
3 files changed