[Reader] Make sure the Application Closed event is correctly tracked

We've observed a downtrend in the application_closed event on iOS. Upon investigation at the backend, it appears events are being rejected due to "Event timestamp older than 14 days." This suggests that the app is not sending analytics events upon closing or backgrounding.

Dev notes: We track this event every time the user backgrounds the app, but we also bulk-send queued tracks events when the app is backgrounded. This means, the application_closed event may miss the current batch due to race condition and will only get sent on the next batch—that is, until the timer fires (every 15 seconds while the app is in the foreground) or until the app is backgrounded again.

Inspect WordPress/Classes/Utility/Analytics/WPAppAnalytics.m and the referenced TracksService.m batching and timer paths. Trace how application_closed is queued when the app backgrounds and whether it can enter the next batch; done means the event is included in the current background send instead of waiting for a later timer or background transition.