Add DPI support for Android and iOS.

For iOS, it is inspired from following SO threads:
http://stackoverflow.com/questions/3860305/get-ppi-of-iphone-ipad-ipod-touch-at-runtime/7922666#7922666
http://stackoverflow.com/questions/13178186/how-can-i-detect-the-dpi-on-an-ipad-mini

For Android, it is inspired from this:
http://stackoverflow.com/questions/3166501/getting-the-screen-density-programmatically-in-android