I have a python3.7/kivy1.11/numpy1.17 program that I transform into an apk with buildozer on ubuntu 19.04, ok it works.
But my program uses a text file for its configuration and that's the problem!
How to link this.txt file to the apk and incidentally where to find this file on the mobile?
Sorry, but Android is still a little bit of a mystery to me.
In my spec file I only mentioned my txt file as 
        "source.include_exts = py,kv,txt"
but of course it doesn't work.
Thank you for any suggestions as to how to proceed.
Buildozer packages whatever folder you tell it, and it's unpacked with the same structure on Android. Just make sure the text file is in that folder, add its extension to source.include_exts (as you already did) and access it the same way you would on the desktop.
Buildozer android and the attached txt files