Trigger face auth whenever user switching has completed.

Summary of changes:
 - Provide API in the repository to pause/resume face auth.
 - To avoid race conditions between trigger and the gating check both relying on the same Flow, move this to the interactor layer

Fixes: 285516490
Test: atest DeviceEntryFaceAuthRepositoryTest
Test: atest KeyguardFaceAuthInteractorTest
Test: verified manually,
  1. Setup multiple users, unlock the device at least once after reboot with main user
  2. Enroll face auth for the main user
  3. Switch to second user.
  4. Switch back to the main user.
  5. Face auth should get triggered right after the switch.
Change-Id: I69e36243ce2ebf45c8e023fdae3a2eb26dbe00d0