What is this `.appodeal` file?
This is generated by the Appodeal advertisement SDK integrated in some of your installed apps, likely to act as some sort of flag.

As for telling which app generated it, aside from going the hard way (installing and starting the apps one-by-one), you should also be able to use tools like MyAndroidTools to check the services and broadcast receivers an app possesses. Apps with ad SDKs will likely have services and receivers dedicated to maintaining the SDK.