[Regional Preference] Should not save "default" into Locale.

Set wrong value in the previous refactory changes, should set valus as null when the default is selected

Bug: 281993323
Test: manual test and QA test
Change-Id: I897791a38a50588d7d721bf57c0d7bdc1734e189

src/com/android/settings/regionalpreferences/RegionalPreferenceListBasePreferenceController.java

@@ -63,7 +63,9 @@ public abstract class RegionalPreferenceListBasePreferenceController extends
             pref.setKey(item);
             pref.setOnPreferenceClickListener(clickedPref -> {
                 setSelected(pref);
-                RegionalPreferencesDataUtils.savePreference(mContext, getExtensionTypes(), item);
+                RegionalPreferencesDataUtils.savePreference(mContext, getExtensionTypes(),
+                        item.equals(RegionalPreferencesDataUtils.DEFAULT_VALUE)
+                                ? null : item);
                 mMetricsFeatureProvider.action(mContext, getMetricsActionKey());
                 return true;
             });