update tutorial and provide a SDK setup tutorial (#2879)

Summary:
Steps to repro app:
1. Follow https://docs-preview.pytorch.org/pytorch/executorch/2817/getting-started-setup.html to set repo and install requirements.
2. If you haven't set up Java/SDK/NDK already, you can follow https://github.com/pytorch/executorch/blob/170956a8d3e7c811bfd788c15676f3a912779510/examples/demo-apps/android/LlamaDemo/SDK-quick-setup-guide.md to set up them. You can also use Android Studio to install them.
3. Follow https://github.com/pytorch/executorch/blob/7ddd415ed9f375908687da910d9dd1feb429f5dc/examples/demo-apps/android/LlamaDemo/README.md to build the app.

Pull Request resolved: https://github.com/pytorch/executorch/pull/2879

Reviewed By: mergennachin

Differential Revision: D55820449

Pulled By: kirklandsign

fbshipit-source-id: 4a4ac4669cad3ed9ce2659654a93e3b3b506a45b