I had your same problem. As you described, UE4 on packaging re-writes the file “OBBDownloaderService.java”.
my workaround was continuosly overwriting the file with the modified one(the modified file had my playstore key in it so it would download the OBB).You should be able to do that manually with continously copying and pasting while packaging.
You can also write a simple application that does that for you every second. I Had made one but i’ve lost source and executable,i’ll let you know if i find it again .