Java FFI on Android

The FFI needed some modifications to work on Android:
* Android does not implement the full JNI (so we deactivate them)
* Its implementation does not expose the same signatures as OpenJDK's
* Provide our own load_jclass because we need a specific class loader (JNI's FindClass is no enough on Android)
* Using Dalvik's long corrupted data going through Java (more debugging will be needed, anyway we can use Ints to hold pointers to C objects)

In the API, intro `App::native_activity` as a shortcut to get the main android Activity.

Update mnit_simple to use Java on Android.