[ntp-journeys] GM3 color adjustments

Adjusts a couple colors on the NTP Journeys surface to match upcoming
GM3 color decisions.

Figma: https://www.figma.com/file/5xWeeSmVQTd4yW3s6aFJ1f/CDDS-UX%2FEng-Spec?type=design&node-id=6065-219186&t=X2XXQ6lk6SPw6AY3-0

- Search icon uses the secondary (or subtle) color as opposed to the
primary color.

- Site icon or globe uses a white background.

Screenshot:
https://drive.google.com/file/d/12Beg9yAF8dGD2e2inV4uWdBPzc80ja7Y/view

Bug: 1430284
Change-Id: I4a03fab84e6ff8eb94d5cd47e009025018f1e774
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4556567


Reviewed-by: Marlon Facey <mfacey@google.com>
Commit-Queue: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148140}