Only fix the list header position if there's no top padding to avoid it shifting around

[kyleve]

• Fix: Only fix the list header position if there's no top padding to avoid it shifting around
• Gardening: Clean up usage of UICollectionView vs List layout context.

In particular, I was messing around with list spacings in the demo app, and noticed that if I apply top spacing to the list, this ends up happening when the header becomes fixed:

https://user-images.githubusercontent.com/327847/182047385-e897dff3-3a0c-4a96-ad9d-eb03100a5e6c.mp4

Which looks sorta odd. I figure if there's any shifting that could occur when changing the list header position, we should fallback to the sticky version, not the fixed version. I think this is probably the best option?

Checklist

Please do the following before merging:

• [x] Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.