devicePixelRatio on Android is not read from the system

On iOS and OSX we our devicePixelRatio() getters return 2.0 in the case of Retina displays. Also on Android, some devices have a of ration != 1, but Qt would always return 1, there.

Here is a list of some ratios: http://www.devicepixelratio.com/

Anyway, IMHO currently it is even good that we always get 1 as result, since there is currently no way to set the devicePixelRatio via API.