chore(auth): update getCurrentUser to check for SessionExpiredException

[phantumcode]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:
Related issue in Amplify UI Android for Authenticator: aws-amplify/amplify-ui-android#145
Description of changes:

• Update getCurrentUser to check and return SessionExpiredException to handle cases where users are signed in but the session has expired.

How did you test these changes?

• manually tested with test app with Authenticator
• verified unit test passed

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)

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