fix(gam): add 1x1 size to all viewports

[miguelpeixe]

Tweaks the approach from #837 so the enforced 1x1 size is not added to the pool of sizes to be calculated for different viewports. The 1x1 should be added to each viewport.

How to test

1. Make sure you have GAM connected and an ad unit configured with multiple sizes (mobile and desktop) to be rendered on a page
2. Visit the page with the ?googfc parameter
3. Confirm the ad unit includes the 1x1 size
4. Switch to a mobile viewport, refresh the page, and confirm the available sizes changes and 1x1 remains