msm_ts: report real pressure not size

android 4 wants real input pressure, not pressure masquerading as size
msm_ts: ignore noisy low pressure touches

msm touchscreen gives noisy x,y positions on light presses. ignore light
presses using an arbitrary cut-off pressure of 5/255. this clipping is
particularly desirable as noisy path start/end data is badly handled by
android 4's VelocityTracker Estimator.
msm_ts: improve zoomhack behaviour when lifting fingers

leave zoomhack enabled as pressure drops (one or both fingers are lifted) and
ignore all subsequent touch events until we get notification of hardware key up
(both fingers lifted). this stops large random jumps in input due to one finger
being raised before the other. zoomhack pinch-to-zoom emulation is still far
from ideal, but this change improves it.