Move face auth related computations to the background

It seems in many places flows were being flowed on the main thread just because the initial registration needed to happen in the main thread. for a couple of places.

This cl makes the combination of flow value happen in the bg thread, while the registration of listeners still happens in the ui thread (just to minimize changes: it would also be possible to change classes to make them thread safe)

This avoids a roughly 30ms of main thread work at every device state change.

Flag: None
Bug: 312894757
Test: DeviceEntryFingerprintAuthRepositoryTest, DevicePostureRepositoryTest + perfetto
Change-Id: If21b2622cf01ffd62c57a5162c65211940c0be72