[Bouncer] Change message to shared flow.

Change to a shared flow so that the flow is not conflated. This will
allow the flow to emit values that are the same as the previous value.

Test: Add face unlock with someone not my face. Fail and disable face
unlock on bouncer.
Fixes: 258160993

Change-Id: Ibc68a23243a5ea88460199de1ea3ee876d658ea1