Should full backup content xml file be empty or not added at all to include all?
Fast Solution:
AndroidManifest.xml
<application
android:allowBackup="true"
android:fullBackupContent="true"
...
...
...
</application>
For more details see: https://developer.android.com/guide/topics/data/autobackup
If you want to silence the warnings, but don't need to exclude any files from the backup, you could also just add tools:ignore="AllowBackup" to the application tag in your AndroidManifest.xml to suppress the warning (or use the Suppress button that you get when viewing the warning details).