Fixed possible null error with SyncService

* Added synchronized to initUserState to make sure this class doesn't become partially initialized that could create null errors.
  - If SyncService.onCreate was started just before OneSignal init was called then the initUserState method might have been run on multiple threads.
* Removed dead code - method updateIdentifier.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java

@@ -470,10 +470,13 @@ void doRetry() {
    static void initUserState(Context context) {
       appContext = context;
 
-      if (currentUserState != null) return;
+      synchronized (syncLock) {
+         if (currentUserState == null)
+            currentUserState = new OneSignalStateSynchronizer().new UserState("CURRENT_STATE", true);
 
-      currentUserState = new OneSignalStateSynchronizer().new UserState("CURRENT_STATE", true);
-      toSyncUserState = new OneSignalStateSynchronizer().new UserState("TOSYNC_STATE", true);
+         if (toSyncUserState == null)
+            toSyncUserState = new OneSignalStateSynchronizer().new UserState("TOSYNC_STATE", true);
+      }
    }
 
    static UserState getNewUserState() {
@@ -643,15 +646,6 @@ static void setSubscription(boolean enable) {
       }
    }
 
-   static void updateIdentifier(String identifier) {
-      UserState userState = getUserStateForModification();
-      try {
-         userState.syncValues.put("identifier", identifier);
-      } catch (JSONException e) {
-         e.printStackTrace();
-      }
-   }
-
    static void updateLocation(Double lat, Double log, Float accuracy, Integer type) {
       UserState userState = getUserStateForModification();
       try {