Add high DPI support to Android

Set devicePixelRatio according to DisplayMetrics.density reported
from the Android SDK, and use it to scale windows and all mouse
and touch input events.

Change-Id: Ie02741caf7940aff98e49440b77e862d49113418
Merge branch 'android-hidpi' of git://github.com/suitabletech/qtbase into android-hidpi