Close sticky notification when the user stops the app / background process by swiping up in running app list

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When this happens, the app is stopped, apparently including background processes. So to avoid confusion, the sticky notification that claims the app is syncing should be removed by the app.

• Research whether there's time to clean up right before the app is fully terminated by the system
• Look into a way to mark the notification so it's automatically cleaned up

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When removing an app from the task list, the background service may continue to function for several minutes or so, but eventually it does seem to clear out, at least on my physical device.

I'll likely have to write a patch according to MaikuB/flutter_local_notifications#207 (comment). I'll have to look into using Service.onDestroy() or a related method to clear out notifications tagged in the flutter_local_notifications plugin.

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See the upstream issue: MaikuB/flutter_local_notifications#2457

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This should be working now.