Bug 1062269 - Load JSON file with shared function in the WhiteList obj

Replace the code for loading a JSON file in the WhiteList object with the new shared function
Don't do the initialization of the WhiteList here, do it in the ``WapPushManager.init()`` function. You can make ``WhiteList.init()`` return a promise and add it to the list of promises we're waiting for here:

https://github.com/mozilla-b2g/gaia/blob/84a5bf6632d559bb690d2134fc4b9ef19d9b78cc/apps/wappush/js/wappush.js#L114