Android Customizable Notification Sound

In Android (not sure about iOS) an app can allow the device to use a custom notification sound. But the operating system itself has no way to set this itself the notifications settings. Some apps, such as GMail and Messenger, have this options. Other apps, such as Clash of Clans, doe not. It would be nice to be able to use one of the downloadable Clash of Clans sound effects as the notification sounds for my game's notifications. Thematic! Satisfying! Good PR for SuperCell! (Yes, there are third-party apps that override all the notification controls and would allow this to happen. But that is the sloppy solution. Cleaner if the Clash of Clans app allowed it the way the operating system expects.)