java ffi: use Java int to store C pointers

While long worked with OpenJDK on linux, it did not on Android.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
android: support FFI with Java

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
android: enable code in AndroidManifest

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro the load_jclass method

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: use load_jclass in Java FFI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/mnit_android: intro App::native_activity

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples/mnit_simple: test Android log and toasts

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>