Add Null check FingerprintSettings#updateAddPreference()

When fingerprint finish enrollment, user click Done
The flow would be:
-> wm_finish_activity
-> relaunch_resume_activity
-> wm_on_destroy_called
-> wm_on_create_called
-> wm_on_start_called
-> wm_on_activity_result_called
   In U, we need to add NPE check when updateAddPreference()
   earlier then addFingerprintItemPreferences()
-> wm_on_resume_called

Bug: 275519315
Test: manual enroll SFPS, in FingerprintEnrollEnrolling,
      rotate device, then complete the enrollment
      verify the flow never crash and user can see enrolled
      fingerprint in FingerprintSettings

Change-Id: Ia262741aeceeb36eefd159ad2ca1c9fa0e061d54

src/com/android/settings/biometrics/fingerprint/FingerprintSettings.java

@@ -593,10 +593,16 @@ public class FingerprintSettings extends SubSettings {
         }
 
         private void updateAddPreference() {
-            if (getActivity() == null) return; // Activity went away
+            if (getActivity() == null) {
+                return; // Activity went away
+            }
 
             final Preference addPreference = findPreference(KEY_FINGERPRINT_ADD);
 
+            if (addPreference == null) {
+                return; // b/275519315 Skip if updateAddPreference() invoke before addPreference()
+            }
+
             /* Disable preference if too many fingerprints added */
             final int max = getContext().getResources().getInteger(
                     com.android.internal.R.integer.config_fingerprintMaxTemplatesPerUser);