[PriceInsights][iOS] Add a color for the graph gradient

This CL adds a custom color for the graph gradient.
screenshot dark mode: https://screenshot.googleplex.com/Yc2qi47ftTtrX8N.png
screenshot light mode: https://screenshot.googleplex.com/BgszMZTsSLv8R3s.png

Bug: b/338136657
Change-Id: Id73c67fcfce06c5592475af2e7106f124e9a613e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5569983
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Gayane Petrosyan <gayane@google.com>
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Cr-Commit-Position: refs/heads/main@{#1307428}