Fixes velocity tracking bug in SwipeDismissLayout.

This is the framework version of ag/682710.
Original change description:
This change adds a missing call to offsetLocation() when tracking
swipe velocities on a SwipeDismissLayout.  This bug was causing
incorrect velocities to be measured which often resulted in an
incorrectly interpreted leftward swipe cancelling the dismiss
gesture.

Bug: 20350515
Change-Id: I4f3e3668a1f9aab963fdfa9095a43f4c5344703f