Android O includes a variety of system changes. The persistent notifications "X is running in the background" or "apps are running in the background" are one of these changes. Unfortunately, there is no built-in option to whitelist apps which are OK.
I have created a new app that aims to remove such a boring persistent notification. Keep the app installed and such annoying notifications will never appear again.
The app is available with the following link:
https://play.google.com/store/apps/details?id=app.rubber.rem...
Comments are welcome.