DISALLOW_CONFIG_MOBILE_NETWORKS for Mobile Network

- Avoid intent start MobileNetworkSettings page
 - Avoid Mobile network preference show on screen

Flag: EXEMPT bug fix
Fix: 289232540
Test: Manual test. see b/289232540#28
Test: atest passed

Change-Id: I25b75673fbc0758dc06ca15f890e5dee0ea1367f

src/com/android/settings/network/SubscriptionsPreferenceController.java

@@ -241,6 +241,8 @@ public class SubscriptionsPreferenceController extends AbstractPreferenceControl
         if (mSubsGearPref == null) {
             mPreferenceGroup.removeAll();
             mSubsGearPref = new MutableGearPreference(mContext, null);
+            mSubsGearPref
+                    .checkRestrictionAndSetDisabled(UserManager.DISALLOW_CONFIG_MOBILE_NETWORKS);
             mSubsGearPref.setOnPreferenceClickListener(preference -> {
                 connectCarrierNetwork();
                 return true;