Extend SwipeHelper to support vertical gestures
* Un-hardcode CONSTRAIN_SWIPE and FADE_OUT_DURING_SWIPE, define
  through proper callback, per-layout
* Up (GESTURE_NEGATIVE) and down (GESTURE_POSITIVE), mostly needed
  by new Hover hide /w swipe up gesture

- @cristianomatos: minor adaptations to fit crDroid sources and heads up

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Change-Id: I68c42a8ccabca73a1b241ba7cf264db9c912fccd