Fix FaceSettings UI problem

Update the visibility of button in onStart to avoid missing check
state if Fragment & Activity were restarted

Bug: 269553342
Test: 1. enroll face
      2. go to settings > Security  privacy > device lock > face & fingerprint unlock
      3. Eenter screen lock
      4. Click face unlock > delete face model
      5.re-enroll face
      6.check the UI of face unlock detailed page
Change-Id: I152467afe2cc90932a53fe73b541e97b5b742831

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

@@ -203,7 +203,11 @@ public class FaceSettings extends DashboardFragment {
         if (savedInstanceState != null) {
             mToken = savedInstanceState.getByteArray(KEY_TOKEN);
         }
+    }
 
+    @Override
+    public void onStart() {
+        super.onStart();
         final boolean hasEnrolled = mFaceManager.hasEnrolledTemplates(mUserId);
         mEnrollButton.setVisible(!hasEnrolled);
         mRemoveButton.setVisible(hasEnrolled);