Set kSecAttrAccessible key to kSecAttrAccessibleAfterFirstUnlock

See: https://blog.loom.com/how-we-fixed-the-ios7-forced-logout-bug-thats-been-plaguing-so-many-apps/

And: http://software-security.sans.org/blog/2011/01/05/using-keychain-to-store-passwords-ios-iphone-ipad/