Bug 977056 - [Dialer] Visual discrepancies: Call ended view under conference call participants list

This should return a ```m``` variable instead, which is initialized to '12:34' by default. e.g.
```js
mCurrentDuration: '12:34',
get currentDuration() {
  return mCurrentDuration;
}
// ...
mTeardown: function() {
  // ...
  this.mCurrentDuration = '12:34';
  // ...
}
```