Use official approved method of getting library folder on iOS. Should…

[verybadcat]

… be more future proof.

[dsplaisted]

@jamesmontemagno Since I'm not an expert on iOS, does this look like a good change to you?

[jamesmontemagno]

@conceptdev? I would assume you would know best or @praeclarum

I am pretty sure what you had is correct as we map down to that.