Move PushNotificationIOS badge writes off the main thread

[fkgozali]

Summary:
PushNotificationIOS.setApplicationIconBadgeNumber can still hang the main thread long enough to trigger an 0x8badf00d watchdog kill, even after the switch to -[UNUserNotificationCenter setBadgeCount:withCompletionHandler:].

That API takes a completion handler but is not asynchronous to its caller. Inside -[UNUserNotificationServiceConnection setBadgeCount:forBundleIdentifier:withCompletionHandler:] it dispatch_syncs onto the UserNotifications connection queue. That queue is serial and shared by every UserNotifications call in the process. A synchronous call on another thread, such as reading UIApplication.applicationIconBadgeNumber, holds it for a full XPC round trip to usernotificationsd. RCTPushNotificationManager.methodQueue is the main queue, so a slow daemon reply on any thread becomes a main-thread hang.

In the hang reports the stacks look like this:

• Main thread: -[RCTPushNotificationManager setApplicationIconBadgeNumber:] -> -[UNUserNotificationCenter setBadgeCount:withCompletionHandler:] -> _dispatch_sync_f_slow / __DISPATCH_WAIT_FOR_QUEUE__
• Another thread: -[UIApplication applicationIconBadgeNumber] -> -[UNUserNotificationServiceConnection badgeNumberForBundleIdentifier:] -> _dispatch_lane_barrier_sync_invoke_and_complete -> xpc_connection_send_message_with_reply_sync

This change issues the badge write from a private serial queue. Serial keeps writes in order, so the last write wins. Utility QoS, because nothing waits on the queue. getApplicationIconBadgeNumber is unchanged: UIApplication.applicationIconBadgeNumber is main-thread-only, and there is no public off-main read API.

Changelog: [iOS][Fixed] Fix remaining main-thread watchdog hang in PushNotificationIOS.setApplicationIconBadgeNumber

Reviewed By: christophpurrer

Differential Revision: D125826470

[meta-codesync]

@fkgozali has exported this pull request. If you are a Meta employee, you can view the originating Diff in D125826470.