Expose `FaceManager` APIs as `@SystemApi`.

Enable privileged apps with `USE_BACKGROUND_FACE_AUTHENTICATION` permission access to the face auth APIs for background face auth.

The first use case is Pixel Health (go/rppg-prd). Design doc: go/pixel-health-face-auth (b/312858933)

Android Feature Request Bug: b/305272520

Bug: 318309705
Test: Unit tests.
Change-Id: I037fbc15353cea6fd8f9a97e1e86e90305716f83