According to the fact that (of course) an app can run on different devices, I'm searching a way to list the hardware buttons available on the smartphone.

Usually Volume, Power are always there. Sometimes a button that launch camera is present, sometimes a button that launch "Google Now" is present, etc... How can my app knows which are available?

Final target for me is to use an hardware button to perform a precise action of my app. I implemented it with volume button, but it works only if screen is on and my app is in foreground, while I need to do that also when screen is off and my app is in background.