Android fast app resume

This PR implements Android Fast App Resume. This means that we greatly reduce how often we need to restore content (textures/shaders) from disk when we are backgrounded and reopened.

This relied on a [fix recently added to Xamarin.](http://forums.xamarin.com/discussion/comment/52186/#Comment_52186)

We also state we support ScreenSize changes as this apparently improves the chances that this is successful (see end of [this comment](http://forums.xamarin.com/discussion/comment/60864/#Comment_60864))

I have tested on android 2.3.4 (Xperia Play) and 4.4.2 (HP Touchpad Cyanogen mod) using the Platformer2D sample, reopening the app is basically instant now! :)

Fixes #2600

Edit: If the build fails on the build bot, it may need its Xamarin.Android updating to at least 4.12.3.