The described method in the readme (copying the apk to /priv-app) does not work anymore on Android 9 due the permission-whitelist enforcing ASOP docs. As a result of this, your phone won't boot if you just copy the apk to priv-app!
As far as I understand, the issue should be solvable by creating a matching priv-app-permission.xml file in /etc/permissions.

I'm willing to contribute the required changes to the readme - however since I'm pretty unfamiliar with the android stack a need a bit more information:

• What would be the package name(s) and their required permissions
• Is there any kind of signature stuff involved?