fix(authenticator): fix signed in state when access token is expired

[phantumcode]

• PR conforms to Pull Request guidelines.

Issue #, if available:
#145
Description of changes:
Update Authenthenticator to handle signed in state with expired access token. If getCurrentUser() fails when in a signed in state, sign out and move state to SignIn.

This is consistent with Amplify UI Swift Authenticator behavior: https://github.com/aws-amplify/amplify-ui-swift-authenticator/blob/6561a42866883bd446f33ddb447215d0357d0e43/Sources/Authenticator/Models/AuthenticatorState.swift#L99-L103

How did you test these changes?

• Updated existing unit test
• Verified with manual test by setting access token expiration to 60 minutes

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(liveness): message, fix(authenticator): message, fix(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.