Limit overscroll stretch for animation

Bug: 192465706

Stretch overscroll was allowing pull distances beyond 1, but
the effect only applied until 1. This meant that as the stretch
relaxed, it would appear stopped at 1 until it dropped below 1.

This CL does two things. First, it limits the pull distance
to 1, so that if it is pulled beyond 1, the distance remains
1 and the spring will immediately retract from 1 with onRelease().
Second, when the fling hits 1, it stops the fling animation and
immediately begins retracting from a velocity of 0.

Test: new tests and manual testing for visual effect
Test: I23cd8990fff24dbef36a93c16c115948d774a557
Change-Id: I06f1533aed0ce24756c535789fdcc7e99c5bfb9b