DO NOT MERGE: AGP integration test.

[cpovirk]

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This detects the problem of https://github.com/google/guava/issues/7769.

To actually merge this, I'd want to:

• Make it work with the version of Gradle we already use, 8.5, or else upgrade the somewhat-standard version of Gradle that's used across a handful of Google project, including ours.

• Integrate it into our CI and release processes, including testing with the version that we're building, as in our Gradle integration tests for our module metadata.

• Put some patterns into a .gitignore file here, similar to what we already have for the previous Gradle integration tests.