Rename Japanese OS X bundle

On OS X our Japanese bundle has ja-JP-mac as locale part while those on other platforms simply have ja. We discussed moving all Mac related things to ja but decided against it as Mozilla is indeed using ja-JP-mac as locale. https://bugzilla.mozilla.org/show_bug.cgi?id=418485 has some, well, interesting details. :)

But what we can and should do is just renaming the bundle to only use ja. That should leave the updater related things and inner workings untouched while it

1. won't confuse OS X users (What the heck is ja-JP-mac? I am looking for the ja bundle.)
2. won't break our website which is currently using one value per locale as far as I can see.