fix(singleton): allow oauth listener to be called with the latest config

[israx]

Description of changes

Amplify.configure will call oAuthListener and it will pass the Auth config. This oAuthListener is responsable for completing an OAuth flow. However when Amplify.configure is called more than 1 time, the oAuthListener will be called only with the first Auth config.

This fix makes sure the oAuthListener is called only and only if a valid OAuth config and oAuthListener are already defined. And ensures the oAuthListener is called with the latest config of Amplify.

• Before Amplify.configure with config1 -> Amplify.configure with config2 -> call oAuthListener with config1

• After Amplify.configure with config1 -> Amplify.configure with config2 -> call oAuthListener with config2

Issue #, if available

https://github.com/aws-amplify/amplify-js/issues/12849

Description of how you validated changes

• smoke testing
• unit tests

Checklist

• [ ] PR description included
• [ ] yarn test passes
• [ ] Unit Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• [ ] Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• [ ] New source file paths included in this PR have been added to CODEOWNERS, if appropriate

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