Move phone app ringtone out of QML

The phone app ringtone has been previously implemented in QML. As such, it cannot be activated by an incoming call while the phone application is not visible. IOW, it cannot react to call state events unless the app is visible. In order to fix this, move the ringtone handling to a Phone app class that controls the ringtone sound effect based on callStateChanged events emitted from the Telephony class.