Android LLAMA demo app build rule (#2406)

Summary:
* Unfortunately, we need to build from ExecuTorch root for Android example, because we need Android toolchain and we can't find required packages like gflags, until we have a better solution.
* Add the CMake rule for Android LLAMA JNI code.
* extension_module depends on in its source.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2406

Reviewed By: larryliu0820

Differential Revision: D54867202

Pulled By: kirklandsign

fbshipit-source-id: 75b11050292ccdcdcd67179ac3c11e4300fd23ff