This never showed up in the docs that I looked at, but I ran the analyzer tool from Android Studio (Code -> Analyze Code -> Run inspections by name) for unused resources. The pane that explains the issues found was very large and so I had to expand it to see it all. At the very bottom it made this suggestion:

<issue id="UnusedResources">
    <option name="skip-libraries" value="true" />
</issue>

This worked! This code block goes into the lint.xml on the main app.