Bug 911055 - [zffos1.1][P3][Dailer]DuT doesn allow to dial a <<pause>>

This whole thing looks really funny and I don't think is a good candidate for recursion. I would write it like this:

```js
function stopDtmfTone(delay) {
  setTimeout(function() {
    telephony.stopTone();
  }, delay);
}

// ...

var accumDelay = 0;
dtmfDigits.forEach(function(dtmfDigit) {
  // Comma is a pause indicator.
  if (dtmfDigit === ',') {
    accumDelay += DTMF_SEPARATOR_PAUSE_DURATION;
  } else {
    setTimeout(function(delay) {
      telephony.startTone(delay);
      stopDtmfTone(delay + DTMF_PLAY_LENGTH);
    }, accumDelay, accumDelay);
  }
});