Bug 899328 - Register only one listener for mozbrowserloadend and appopen on a given app iframe.

This prevents us from leaking listeners.
Bug 858463 - Hide the subtitle Accounts if there is no account in settings (cherry picked from commit c542dd1)
Merge pull request #11238 from jlebar/apploaded-listener-b2g18

Bug 899328 - Register only one listener for mozbrowserloadend and appopen (v1-train PR)