[flexiglass] Adds authmethod flow.

Now that I realized that we _can_ listen for changes to the
authentication method (using a broadcast), we can introduce this very
useful flow to our system. This allows downstream collectors who
previously ignored auth method changes to pick up on these.

Bug: 292502433
Test: manually verified that authentication method changes are picked up
by Flexiglass.
Test: unit tests updated.

Change-Id: Ifbbf5be25ed6bdb49f6c8793f5b5093fb14679bf