Fixes for Android Depth/Stencil buffer problems.

It appears on some newer devices (Nexus 4) the defaults used by the AndroidGameView result in a surface without a depth or stencil buffer. This PR corrects the issue by attempting to use the PreferredDepthStencilFormat to pass in the correct parameters that the user wants. 

 Fixed an issue in AddSpecular where on certain android gpu's applying

the color.a to the specular wipes out the texture.

The fix is to apply it in two operations.

This has been tested on 4 different devices and emulator and works ok. 

If we had the xamarin test cloud we could know for sure but hey ho ;)