Swipe: make swipe thresholds uniform across devices

Devices with high screen densities require very small swipes to activate
a key. Make the swipe distance uniform across devices by using the screen
density from ro.sf.lcd_density and activating a key based on the following
thresholds:
  Left/Right swipe: 0.5in
  Up/Down swipe: 0.3in

Change-Id: I39fe4b1007a4e4784d3d7d26a392d78b1adf9462