AccountService: Implements default account failsafe

Implements one of the fixes outlined in issue #2637.

/cc @sendhil + @astralbodies + @koke 

@koke: We've been bothering @aerych, and he kindly pointed out that the `NSEntityMigrationPolicy` instance might change, and this was one of the issues he faced when he implemented a Policy subclass. That would be the reason why we use NSUserDefaults.

(Thanks @aerych!!)