Fix concurrent issue

- Add a thread lock

Fix: 286495550
Test: make pass
Change-Id: I72877551bda18cf93734d270770a0bd53bab465b

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

@@ -295,7 +295,9 @@ public class MobileNetworkRepository extends SubscriptionManager.OnSubscriptions
     }
 
     public void removeRegister(MobileNetworkCallback mobileNetworkCallback) {
+        synchronized (this) {
             sCallbacks.remove(mobileNetworkCallback);
+        }
         if (sCallbacks.isEmpty()) {
             mSubscriptionManager.removeOnSubscriptionsChangedListener(this);
             mAirplaneModeObserver.unRegister(mContext);