Android: Implemented Texture2D.SaveAsPng and SaveAsJpeg

Also implemented GetData<int>() to make it possible to pass int[] to
Android APIs.

To see an example of it working, check out this repo: https://github.com/jonathanpeppers/MonoGameSaveImage

The sample you can tap the screen, and it will launch an Android intent to open the saved image. You can comment/uncomment code to try JPG or PNG.