How can I keep an app (not made by me) running in the background continuously Is it possible to keep an app (not developed by me) running in the background. I know that it is possible to keep an app persistent in background by making it a service, but since I am not the developer of the app I want to keep active, is there a way to do this through some settings changes?
Android kills apps automatically for memory. However there is an internal "priority" which it follows in doing so. For ex the system apps have usually much higher priority and hence lesser chance to be killed.

There is a way to explicitly assign higher priority to any app using third party apps. One such app is Memory Locker.

Just install and open the app, tap on app you want to be locked in memory, and set the priority. The lower the priority the lesser is the chance for that app to be killed or suspended.

NOTE: It needs root.

Additional Info:

How it works:
It actually edits a file called oom_adj inside /proc/[process ID]/. This file is nothing complex. Infact, it can simply be opened in any text editor and it contains just a number (for example -12). This number is process priority. You can change/edit this manually and set the priority for processes yourself if you dont want to use external app.