Play a sound once a call is finished

This pull requests works towards solving this ticket: https://github.com/WhisperSystems/RedPhone/issues/299
I basically copied the code from notifyBusy() as the busy sound is the same as the call-end-sound and as far as i understood it delays the termination of RedPhoneService the same way and should not create any other impact. 

But of course i'm open to suggestions. 

Best regards
WhiteHatTux