Update screenshot UI to use new Material colors

Also updates some of the padding/margins and makes the dismiss
button 32dp wide (tappable size 48dp).

https://screenshot.googleplex.com/6QrPRKuJX7ruAZA (light mode)
https://screenshot.googleplex.com/6CwbfTCdUU8Jmrx (dark mode)

Bug: 187967953
Fix: 187967953

Test: manual
Change-Id: Ice0109633810b4de0aeaae9024fe4257bc2fd4c4