Linearly complete overscroll stretch when close

Fixes: 190475978

Damped spring animations asymptotically approach their destinations.
The final pixels can take many frames to complete, but stopping them
means that we get a disruptive jump in those final pixels.

This CL detects when the edge effect animation is close to the finish and
completes the animation with a linear velocity that matches a
common velocity at 8 pixels distant. This means that the animation
terminates quicker so that touch events directed at the contents
after the animation completes (e.g. taps) can interact with the contents
soon.

The CL also adjusts the delta for detecting zero in the stretch
animation as it was jumping by a pixel or more at the end of an otherwise
smooth animation.

Test: manual testing for the visual effect
Change-Id: Ie249b0265c5c5939b597668d5afe4f76d0430821