Fix the incorrect touch_cnt.

This fixes the bug which decreased touch_cnt into -1 inside LowPressureFilter.
For example, a low-pressure event with touch_cnt of 0 and finger_cnt of
1 will trigger this bug.

BUG=chromium-os:27368
TEST=unittest; see if we could pass the unittest.

Change-Id: Ice8d0d540984d2460952ef2746f2d21313f809e1