Call stopForeground when all downloads are completed

[Amejia481]

To be able to have background privileges on AbstractFetchDownloadService, we have to adhere to the rules of Foreground services, one of these rule indicates that a foreground service need to have a pinned notification to make users aware of the app doing something in background, while we have these privileges the notification can't be dismissed or swipe away. This what users are experiencing in issues like mozilla-mobile/fenix#15527, this is not a problem while downloads are being downloaded as we don't want users swiping them away by mistake, BUT when downloads are completed we need to call stopForeground and allow users to swipe the foreground notification away.

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