Remove flatDir section from build file and the aars directory #16147
Description
flatDir is deprecated and shouldn't be used because:
WARNING:Using flatDir should be avoided because it doesn't support any meta-data formats.
We use this for screenshot generation that's introduced in #12812. It looks like the documentation has been updated so as not to require the aar file to be added manually anymore: https://firebase.google.com/docs/test-lab/android/instrumentation-test, so we can hopefully remove the artifact. Alternatively, we can manually publish the aar file to our S3 Maven and deal with the issue that way.