Ack, wrap advisePersistThreshold() in lock.

Bug: 6449725
Change-Id: I702dc2ff9d4b237c46d89bcae41289bc3e9ef2f8

services/java/com/android/server/net/NetworkStatsService.java

@@ -616,10 +616,12 @@ public class NetworkStatsService extends INetworkStatsService.Stub {
         // persist if beyond new thresholds
         final long currentTime = mTime.hasCache() ? mTime.currentTimeMillis()
                 : System.currentTimeMillis();
+        synchronized (mStatsLock) {
             mDevRecorder.maybePersistLocked(currentTime);
             mXtRecorder.maybePersistLocked(currentTime);
             mUidRecorder.maybePersistLocked(currentTime);
             mUidTagRecorder.maybePersistLocked(currentTime);
+        }
 
         // re-arm global alert
         registerGlobalAlert();