Fixed FaceEnrollIntro crash.

Test: Manual.
Fixes: 339306372
Change-Id: Ib6b98f78b734a531378c1162eb86725995f45799

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

@@ -170,6 +170,8 @@ public class FaceEnrollIntroduction extends BiometricEnrollIntroduction {
             infoMessageRequireEyes.setText(getInfoMessageRequireEyes());
         }
 
+
+        if (mFaceManager != null) {
             mFaceManager.addAuthenticatorsRegisteredCallback(
                     new IFaceAuthenticatorsRegisteredCallback.Stub() {
                         @Override
@@ -186,6 +188,7 @@ public class FaceEnrollIntroduction extends BiometricEnrollIntroduction {
                             onFaceStrengthChanged();
                         }
                     });
+        }
 
         // This path is an entry point for SetNewPasswordController, e.g.
         // adb shell am start -a android.app.action.SET_NEW_PASSWORD