Adding in Clear Credential Implementation

This will add on the ability to clear credentials to the play auth
module implementation.

Relnote: """Added more generic Void? type rather than Void across java
  and kotlin for clearing credentials, specifically for use in the
  callback for clear credentials"""
Bug: 258061894
Test: WIP JUnit Tests, design concept only for now (builds fine)
Change-Id: If61f4f3c98e4e13129ca722eee9e953306f19e56