How do I keep apps from running in the background on Android?

You can detect currently foreground/background application with ActivityManager. getRunningAppProcesses() which returns a list of RunningAppProcessInfo records. To determine if your application is on the foreground check RunningAppProcessInfo.

You can allow a certain app to run in the background by going to Settings > Apps & notifications > Advanced > Special app access > Battery optimization . This will initially show you the apps which are Not Optimized – that is, the ones that can run in the background even while the phone is sleeping.

Stop Apps From Running in the Background on Android