AMP-CONSENT v2 UI Followup Design Changes

[torch2424]

relates to #18828
relates to #19841

cc @zhouyx

After (UI/UX) design review with @spacedino , we came to the following remaning UI / UX things we should address before completing the AMP Consent v2 UI/UX:

• Implement Nicer Example in diy-consent.html
• Fix the Flash of the "teal" background in diy-consent.html
• Compensate for the iOS Safari Nav Bar.
  • If easy fix, have the consent bound to the bottom of the nav bar, and raise and lower with the nav bar.
  • If not easy, make the height calc(30vh + /*iOS Bottom Nav Bar Height*/), and document for CMPs
  • Update diy-consent.html to show this behavior
• Add drop shadow to the loading placeholder bottom footer thing (same shadow from sticky ad)
• Add 8px Rounded corners

[ampprojectbot]

This issue doesn't have a category which makes it harder for us to keep track of it. @torch2424 Please add an appropriate category.