Fixed JNI on device

I had to change a few things in runtime/JniInterface.cs so it works with the AOT compiler. The AOT compiler needs hints to compile trampolines for methods that are called from native code. For that, a custom attribute called MonoPInvokeCallback has to be added to each method of JNIEnv.

I also had to add monotouch.dll as a reference to the IKVM.Runtime.JNI target in the runtime/runtime.build file. I reference that via an absolute path, ymmv.

Tested all of this by running one of our libgdx demo games which exercises all the JNI functionality.