WIP: Add platform independent high-dpi support to QtGui

Add coordinate scaling support to the QWindow/
QWindowSystemInterface layer. The scale factor can
be set with the QT_HIGHDPI_SCALE_FACTOR environment
variable.

Setting a scale factor different than the default (1)
now has the following effects:

QWindow::devicePixelRatio is set accordingly, enabling
the high-dpi code paths.

QWindow and related classes now return geometry in
device independent pixels. This includes screen,
desktop and window geometry as well as event coordinates.

The platform plugins continue to operate in device
pixels, unaware of the scaling.

Task-number: QTBUG-38858
Change-Id: I63f04475f29dd025ec95d1e56ba7f3f5f1ea726d
Add high DPI support to Android

Set QHighDpiScaling's scale factor to DisplayMetrics.density reported
from the Android SDK