fix(messaging, ios): serialize access to background handler state

this should make race conditions impossible, in case that was the cause
for a recent crash

note that the 25 second timeout block did *not* execute in testing if
the completion handler was called - the app went back to sleep immediately,
so no create_dispatch_block / dispatch_block_cancel was needed

Author: mikehardy

packages/messaging/ios/RNFBMessaging/RNFBMessaging+AppDelegate.m

@@ -163,14 +163,16 @@ - (void)application:(UIApplication *)application
       // If app is in background state, register background task to guarantee async queues aren't
       // frozen.
       sharedInstance.backgroundTaskId = [application beginBackgroundTaskWithExpirationHandler:^{
-        if (sharedInstance.backgroundTaskId != UIBackgroundTaskInvalid) {
-          [application endBackgroundTask:sharedInstance.backgroundTaskId];
-          sharedInstance.backgroundTaskId = UIBackgroundTaskInvalid;
-        }
+        dispatch_get_main_queue(), ^{
+          if (sharedInstance.backgroundTaskId != UIBackgroundTaskInvalid) {
+            [application endBackgroundTask:sharedInstance.backgroundTaskId];
+            sharedInstance.backgroundTaskId = UIBackgroundTaskInvalid;
+          }
+        };
       }];
 
       dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(25 * NSEC_PER_SEC)),
-                     dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
+                     dispatch_get_main_queue(), ^{
                        if (sharedInstance.completionHandler) {
                          sharedInstance.completionHandler(UIBackgroundFetchResultNewData);
                          sharedInstance.completionHandler = nil;

packages/messaging/ios/RNFBMessaging/RNFBMessagingModule.m

@@ -220,7 +220,7 @@ - (NSDictionary *)constantsToExport {
 }
 
 RCT_EXPORT_METHOD(completeNotificationProcessing) {
-  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
+  dispatch_get_main_queue(), ^{
     RNFBMessagingAppDelegate *appDelegate = [RNFBMessagingAppDelegate sharedInstance];
     if (appDelegate.completionHandler) {
       appDelegate.completionHandler(UIBackgroundFetchResultNewData);
@@ -230,7 +230,7 @@ - (NSDictionary *)constantsToExport {
       [[UIApplication sharedApplication] endBackgroundTask:appDelegate.backgroundTaskId];
       appDelegate.backgroundTaskId = UIBackgroundTaskInvalid;
     }
-  });
+  };
 }
 
 RCT_EXPORT_METHOD(requestPermission