Implement AuthTokenHandler enum.

This CL implements AuthTokenHandler enum which is used to manage
different states of an auth token during the operation life cycle and
also used by the authorize_* methods of enforcement module to return
the status of the auth token.

Bug: 159461976
Test: TBD
Change-Id: I0c6fc14ece65bc3c161c3403dcb6bbd167780713
2 files changed