Fix demo app workflow and tutorial (#1979)

Summary:
Now we use sources from extension/android, and we need to update the instructions for Android demo app.

Note: We have two ad-hoc solutions

1. For Java, so far we are copying sources; we should use package instead.
2. For JNI, we are building from top-level CMakeLists.txt; we should build from extension/android and `find_package()` for deps.

We will address these two issues in follow-up PRs.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1979

Reviewed By: mcr229

Differential Revision: D53791261

Pulled By: kirklandsign

fbshipit-source-id: 73e48f1cc3c444426b8b3252fc48762aa182cb98