palm_classifying_filter: Simple logic to detect fat finger

We add a simple logic for reclassifying a previous palm as a fat finger:
if the finger's lifetime max pressure is less than
palm_pressure_ * fat_finger_pressure_ratio_ (default 1.4)  and
the finger's lifetime max width is less than
palm_width_ * fat_finger_width_ratio_ (default 1.3)
and the finger has moved more than fat_finger_min_dist_

This is mainly for making a flat thumb to move the cursor. But we
set the controlling parameters very conservative so that the real
palm detection does not regress.

BUG=chrome-os-partner:13144
TEST=1. Manual test on Link & Snow, no obvious regression in palm
        detection, and flat thumb can move the cursor (to a certain
        degree)
     2. Regression test snow2.3/flat_thumb_move passes.

Change-Id: Idf442807b95730cafb4698c5ee1eded39997e6c7
Reviewed-on: https://gerrit.chromium.org/gerrit/47369
Commit-Queue: Yufeng Shen <miletus@chromium.org>
Reviewed-by: Yufeng Shen <miletus@chromium.org>
Tested-by: Yufeng Shen <miletus@chromium.org>