chore(auth): refractor handleSignedIn method in AuthenticatorViewModel

[edisooon]

• PR conforms to Pull Request guidelines.

Issue #, if available: https://github.com/aws-amplify/amplify-ui-android/issues/189

Description of changes:
instead of "manually" moveTo SignIn state after signing out, we could listen to HubEvents: AuthChannelEventName.SIGNED_OUT, which was implemented in start() method of AuthenticatorViewModel.kt.

How did you test these changes?
(Please add a line here how the changes were tested)
Instead of testing the resulting state (which should be SignIn), we just need to make sure that signOut() is being called when getCurrentUser errors with SessionExpiredException during start

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.