Note => This app has been tested on Google Nexus 6 only, it might work on other devices but there is no garantee.
Please let me know if it works on your device so I'll add it in a list in this description for other users. Thank you!
★ Only one requirement to work ▶ Yuni Ringer needs notification access
★ No need for app to stay opened ▶ Run it once and that's all you can exit
★ No need for a rooted device ▶ Only public API is being used
★ No need for a foreground service ▶ No icon always displayed in status bar
★ Application autoruns at boot time ▶ No need to relaunch it after reboot
This application is free with no ads but I spent days to read Android source code to find out a solution by using public API only (no root nor java reflection), so try it on your device, and if satified, if you can and if you want, you can support me by purchasing my gratitude ^^