[Android] Fix demo app workflow and tutorial

[kirklandsign]

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.

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit a32736b with merge base b16ea47 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for extension/android/CMakeLists.txt:

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[facebook-github-bot]

@kirklandsign has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@kirklandsign merged this pull request in ac9c121.