Thanks, Steve!
I'm not very familar with the Android file system (may be this is even typically for Linux?!).
In the meantime, I am using the SD card of my tablet, which works fine.
I thought, if someone would like to use my apk and does not own a mobile or table pc including external memory, I would have to store such a file to internal memory.
The file should be configurable by the user, since it keeps stuff like IP address, port number etc.
Thanks also to agraham & monster67!
Harald