This CL adds a subtitle as a header in What's New for Price Insights.
In addition, it removes WhatsNewTableViewFakeHeaderItem, which was
no longer used.

Screenshot: https://screenshot.googleplex.com/5vJ85QMiHAAEqHP.png

Bug: chromium:
Change-Id: I598f92edbd3078aa79c0f045b90eda72856be075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6002529


Reviewed-by: Gayane Petrosyan <gayane@google.com>
Commit-Queue: Cheick Cisse <cheickcisse@google.com>
Cr-Commit-Position: refs/heads/main@{#1380865}