Bug 1062805 - Used the shared JSON file load helper in the ringtones app R=Goutam

:julienw is right, the issue here is that in your ``then()`` call you're returning a new Promise (that's implicit to the ``then()`` function) but that promise is not returned from the outer scope so the caller times out as it doesn't receive anything.

The solution is to do exactly as :julienw suggested, remove the outermost promise and then rewrite the code as in the comment above. This will make ``getSoundFilename()`` always return a promise so that the caller can chain to it correctly.