[NTP] Fix ricker picker dimensions to match spec

Adjust positioning of elements to match spec:
- Reduce width of dialog to 568px to match spec.
- Remove hidden spacing between some inline-block elements due to font
  size.
- Simplify header by removing some extra div wrappers.
- Hide the back button using "display: none" in order to avoid messy
  margin adjustments.
- Remove side margin of the last background tile per row.
- Align footer and daily refresh elements using flexbox instead of
  inline-block.

Misc. fixes:
- Hide daily refresh toggle when background image panel is not open.
- Fix back button focus color in dark mode.
- Hide scrollbar when not required.

Screenshots: https://drive.google.com/open?id=1p5VB83jYaUqk0Pz3mzlO8NTKbd9dgiDG

Bug: 987076
Change-Id: I12d9132f66361029b1b644a91f9de639ee74c2d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721375
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682037}