Retain a notification observer in StatsForegroundObservable

[crazytonyli]

Relates to #20994.

The removeWillEnterForegroundObserver function removes the wrong observer. This PR fixes it by adding the correct notification observer as an associated object, so that it can be removed later.

I want to avoid using Objective-C runtime API, but I don't think there are other viable alternatives here. We can't use the selector API because we can't add a @objc function to an extension.

Regression Notes

1. Potential unintended areas of impact
   None.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist: N/A

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[mokagio]

I want to avoid using Objective-C runtime API, but I don't think there are other viable alternatives here. We can't use the selector API because we can't add a @objc function to an extension.

I think the whole design here could do with a overhaul. It doesn't seem appropriate to use a protocol in this way. Even the where Self: UIViewController extension is odd because none of the logic it adds has anything to do with UIViewController.

I have an idea for what should be a quick update. Would you be okay with me having a go instead of merging this?

I'm going to approve in the meantime, so if I haven't followed up by the time 22.8 should code freeze, we can get this fix it.

[crazytonyli]

I have an idea for what should be a quick update. Would you be okay with me having a go instead of merging this?

@mokagio Absolutely. I have added a "do no merge" label to this PR for now, while waiting for your changes.

[mokagio]

I'm going to approve in the meantime, so if I haven't followed up by the time 22.8 should code freeze, we can get this fix [in].

I didn't get to this. I'll follow up eventually because I have opinions 😅 but that shouldn't delay this fix.