Skullgirls does allow you to background the app and return to it as long as your device has enough memory to support Skullgirls and whatever app you're using along side it.
For example, if Skullgirls is using 1GB of memory, and you accept a phone call, and your phone call app uses another 1GB (unlikely) of memory, now you're using 2GB of memory total. If your device only has 1.5GB of memory, then your operating system (Android) will forcibly close the Skullgirls app to make room for the phone call.
Unfortunately, we can't prevent this closing logic.
The operating system (iOS or aOS) decides when suspended apps need to forcibly close because they're taking up too much memory and new apps are requesting it.