how to listen to hardware button when the screen is off flutter

Question

I want to make a walkie talkie app that can run even while the screen is off using flutter. Currently i can play the audio even on lockscreen by keeping the app alive. But i want to assign volume up button to be a PTT button. So user can hold the volume up button to start the conversation. Can i do that on flutter?

currently i'm accessing volume button like this

class MainActivity: FlutterActivity() {
     private lateinit var channel : MethodChannel;

    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        super.configureFlutterEngine(flutterEngine)
        channel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "mychannel")
    }

    // using platform-specific events
    override fun onKeyDown(keyCode: Int, event: KeyEvent?): Boolean {
        when (keyCode) {
            KeyEvent.KEYCODE_VOLUME_DOWN -> channel.invokeMethod("volumeBtnPressed", "volume_down")
            KeyEvent.KEYCODE_VOLUME_UP -> channel.invokeMethod("volumeBtnPressed", "volume_up")
        }
        // return true means "prevent default behavior", so volume doesn't change and volume bar doesn't appear
        return true
    }
}

but it only works on foreground, and sometimes it gives me an Unresolved reference: KeyEvent