Update Keychain code
Update Keychain code

Summary:
- We were forgetting to pass in a `kSecAttrService` value so we were getting other internal keychain items. This was causing a variety of weird issues.
- Refactor code to ensure that we *always* scope the item class and the service correctly.

This will effectively unlink users in previous versions of the app. Since we're still in beta, this is acceptable.

Test Plan: Manual

Reviewers: wkiefer

Reviewed By: wkiefer

Subscribers: chris

Differential Revision: https://tails.corp.dropbox.com/D148407

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