Don't use kSecAttrAccessible to search keychain

On 3.8.3 we were storing passwords with
`kSecAttrAccessibleAfterWhenUnlocked`. Even if the keychain migration
fix should turn these into `kSecAttrAccessibleAfterFirstUnlock`, it runs
async so the first launch after an upgrade it would appear that the
credentials were invalid.

fixes #306
Merge pull request #390 from wordpress-mobile/306-migration-fix

Don't use kSecAttrAccessible to search keychain