Add helper to determine if touch id is available

You can take advantages of `NSClassFromString()` or something like that check whether it returns nil. Leave a TODO to remove it when we make the change. Ideally we would have Jenkins to have 2 xcode running at the same time (just in case)... I will arrange a time to talk to @makdad about that. Up to you, unfortunately, the soon I merge this, it will break Jenkins when you change the sha in mobile-client.