[iOS] Fixed Incorrect Trackable Section Subtitle Being Displayed

Currently, the Mobile Bookmarks link is appearing in the Trackable
Section's subtitles whenever the user navigates to a webpage that
contains an item they are already pricing tracking. The expected
behavior is for this bookmark link to appear only immediately after the
user has price tracked an item.

In this CL, a new boolean was added to indicate whether the user has
executed a "track item" action over the life time of the price tracking
UI. This boolean value is then used to determine which string is set as
the trackable section header's subtitle. In addition, TableView's
unittest were modified to ensure the proper subtitle is being displayed.

Fixed: 1421081
Change-Id: I5285e46c18e2a5d7a3543ebb52dbe9f540ad80f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4303869
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Daniel White <danieltwhite@google.com>
Cr-Commit-Position: refs/heads/main@{#1112806}