[Bouncer] Revert from shared flow...

back to a state flow with a timestamp. It seems like shared flows replay
the value every time and there isn't a way to propagate one off events.
I am simulating one off events by using a state flow with a unique
timestamp to ensure the event emits.

Test: Added unit tests
Test: fail face unlock until message appears. Unlock phone and restart
screen. Observe that "Enter your pin" is the initial message.
Fixes: 258775372

Change-Id: I6c9fb9a2c79aed9300e9b102a45c9d4c0456536e