ImmediateInterpreter: Make 3f swipes smoother.

Historically for any instantaneous time, we'd take the max movement of
the fingers and use that for the gesture. With this CL, we require
that all three fingers be moving in the same direction (non-zero), and
we average the three values.

BUG=chromium-os:36791
TEST=no regressions, unittests pass, dave moore tested and is happy

Change-Id: I42c23e68c4c5a7ed1b29beadc5de283f78f4075c
Reviewed-on: https://gerrit.chromium.org/gerrit/38931
Reviewed-by: Dave Moore <davemoore@chromium.org>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Ready: Andrew de los Reyes <adlr@chromium.org>