atraczyk authored Dec 06, 2016 and Andreas Traczyk committed Jan 30, 2017

- adds ringtone file to assets directory

- adds the ability to play and stop/restart a ringtone

- adds a toast pop notification on incoming call using
  background audio

- adds a background tasks to respond to call rejection

Change-Id: Ica7195fa0fa32dc10d9207399d98bed518373d15
Tuleap: #790