Update Settings together with generateChallenge/revokeChallenge

The internal implementation of generate/revoke in system_server is now
asynchronous. To keep existing clients working, the manager classes
introduce a blocking version of the generateChallenge calls. This change
updates Settings to use the backward-compatible blocking calls.

Bug: 157790417

Test: Enroll fingerprint/face
Test: After enrollment, toggle setFeature or do subsequent enrollment
      in face/fingerprint settings
Change-Id: Ib4dfdc5f12530b938ab9b1745f5a19cd9e2eceee

src/com/android/settings/biometrics/face/FaceEnrollIntroduction.java

@@ -176,7 +176,7 @@ public class FaceEnrollIntroduction extends BiometricEnrollIntroduction {
         if (mFaceManager == null) {
             return 0;
         }
-        return mFaceManager.generateChallenge();
+        return mFaceManager.generateChallengeBlocking();
     }
 
     @Override