Enable TN AF for upgrading clients

... in my humble view, (could be) a (proper) fix for #2850

Update:  The second iteration of the MR adds in draft a user prompt UI (which hopefully could facilitate discussion and move things forward):

Post-upgrade, a snack bar will be shown regarding updated AFs:

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which will be dismissed when the user navigates to the Settings tab (or by clicking on the "Review" button to the same effect), when they'll see a card shown at the top which invites them to enable the new TN AF:

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(The "Learn more" button points to the great explainer post by Licaon on the subject)

You may feel free to download the CI build to try it out yourself and see it in action:

• on first launch, you should see... nothing (if one checks out the AF setting, one should see that the TN AF is enabled, which is the expected behaviour for new installations)
• I've included a 'sidekick' (for debug builds only) just to make lives easier for testing: you may long press the AF setting and it'll reset an internal state counter and disable the TN AF to simulate pre-upgrade client position
• Restart and you should see the post-upgrade magic kicks in

I reckon I'm probably not a great UI- / copy- writer, so any review and feedback is welcome!