[NTP] Add main backgrounds page to the richer picker

Add a backgrounds page that is shown by default, and shown/hidden
as appropriate when various options are clicked. Populate it with
some tiles.

LTR: http://go/scrcast/NTAyNzAzNTc0MzI1NjU3Nnw4ZGM5MjRiOS1jZg
RTL: http://go/scrcast/NTI0OTQ5Mzg0Mjc4ODM1MnxiNjJlMzhjYS1lYQ

Bug: 937570
Change-Id: Ib3ab9b5018051640e20d319c763676b2416c76f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599743
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658127}
5 files changed