Bug 1022941 - [email] On language change, cached timestamps do not change to new language

This is the right place to do this, but let's take this opportunity to clean up this code a bit. 

From what I can tell, the window.addEventListener for 'message' is really just looking for a visibilitychange event. So, lets create a named function in this area that does the work that  visibleAppUpdatePrettyDate does (except not all the data checking, just from clearTimeout down), then pass that named function to mozL10n.ready and to a `document.addEventListener('visibilitychange', ...)` call instead of this 'message' listener.