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
diff --git a/packages/Shell/AndroidManifest.xml b/packages/Shell/AndroidManifest.xml
index 3dfc454..61e0ca8 100644
--- a/packages/Shell/AndroidManifest.xml
+++ b/packages/Shell/AndroidManifest.xml
@@ -562,6 +562,9 @@
     <!-- Permission required for CTS test - android.server.biometrics -->
     <uses-permission android:name="android.permission.MANAGE_BIOMETRIC_DIALOG" />
 
+    <!-- Permission required for CTS test - android.server.biometrics -->
+    <uses-permission android:name="android.permission.USE_BACKGROUND_FACE_AUTHENTICATION" />
+
     <!-- Permissions required for CTS test - NotificationManagerTest -->
     <uses-permission android:name="android.permission.MANAGE_NOTIFICATION_LISTENERS" />