Add provider facing GET APIs for Credential Manager.

Credential Manager is a set of APIs that allows developers to get and
create credentials. These credentials are sourced from providers that
are registered with the android framework.

Test: Built and Deployed on device locally.
Bug: 247545196
CTS-Coverage-Bug: 247549381
Change-Id: If5b6569810be30ec6f7997e9161fe9b85d286cfc