Google Play states that my app is not designed for tablets: \"Your APK should include custom drawables assets for common tablet screen densities.\" It links to 4. Use asset
For anyone struggling with this, I submitted the Design for Tablet Contact Form, which you can use to let Google know why you believe your app is designed for tablets (even though it is not labeled as such). Google reviewed my submission and agreed, and the app is now labeled as designed for tablets.
Before submitting the form, you should make sure that your app meets the Tablet App Quality Checklist first.