Bug 1057198 - [LockScreen] Make LockScreen in an iframe before make it as an app

Let's hide all of this detail in lockscreenWindowManager + lockscreenWindow.
```js
getLockScreenContainer: function() {
  return (window.lockscreenWindowManager && window.lockscreenWindowManager.getInstance() && window.lockscreenWindowManager.getInstance().getNotificationContainer()
};
```