gesture: add absolute threshold for start of tap for battery
gesture: add absolute threshold for start of tap for battery

Add absolute threshold on delta_z_inner for tap for battery to guarantee
that we have at least some minimum absolute change in z that has started
the tap for battery.

This change makes it such that if the accel readings are really steady
for a while, then a sudden noisy signal doesn't trigger tap for battery.

BUG=chrome-os-partner:34592
BRANCH=samus
TEST=use tapinfo on to observe tap values and do a double tap of varying
intensities. note that it should be just a little less sensitive to weak
taps.

Signed-off-by: Alec Berg <alecaberg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/237666
Reviewed-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
(cherry picked from commit ae82f824abcd6cd0444034feb6a10c09f8f5e61d)
Signed-off-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

Change-Id: I95367bc7f99b888e15e9ac3a2bc8c9cca32d30d6
Reviewed-on: https://chromium-review.googlesource.com/238063
Tested-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Trybot-Ready: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Reviewed-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Commit-Queue: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec