Is there a way to make an android phone only run one app and nothing else? I want to take an old android phone and turn it into a dedicated calculator. In other words, I'd like to make it so I boot up the phone and my calculator app is running, and there is no way to access any other app, the home screen, app drawer, anything. No lock screen either. Just this app and the settings app, which there would be a shortcut too in this app. 

Is this possible? Is android the best OS for this project? Is there an easy way to make this happen?
If you didn't have the requirement of being able to access the settings (the settings app is part of the launcher in some ROMs), this would probably be easier. In that case, you could simply build a custom ROM that boots straight to calculator. The main disadvantage would be the absence of even a battery indicator.