ImmediateInterpreter: Don't dead reckon small scrolls.

We have code that will detect a possible finger leaving, and at that
time, will continue a scroll in progress by dead reckoning. This is
great for big, intended scrolls, but when they are very small, they
should be surprssed, as they are probably just wiggle as the finger
leaves the pad. In this case of small scrolls, we clear the buffer, so
that we don't get a fling after some effectively stationary time.

This will only apply to boards that opt-in with a given threshold, so
there's no behavior change to existing configurations.

BUG=chromium:424658
TEST=Swanky regression tests; swanky manual testing

Change-Id: Ic1d23a66ece85f59d28841951ae25755b6dd0b58
Reviewed-on: https://chromium-review.googlesource.com/225454
Reviewed-by: Dennis Kempin <denniskempin@chromium.org>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Queue: Andrew de los Reyes <adlr@chromium.org>