ensure user defaults is synchronized after setting avlues (#1646)

Author: charlotteliang

Firebase/Messaging/FIRMessaging.m

@@ -484,6 +484,7 @@ - (void)setAutoInitEnabled:(BOOL)autoInitEnabled {
   BOOL isFCMAutoInitEnabled = [self isAutoInitEnabled];
   [_messagingUserDefaults setBool:autoInitEnabled
                            forKey:kFIRMessagingUserDefaultsKeyAutoInitEnabled];
+  [_messagingUserDefaults synchronize];
   if (!isFCMAutoInitEnabled && autoInitEnabled) {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"