Add option to remove price tracking promo from magic stack

A new card is being added to the magic stack to encourage price
tracking users from other platforms to use the feature on Chrome
for iOS. This CL gives the option for the user to remove the
card from the magic stack (as is the case for many other cards).

code. Overall coverage for the feature is high (see
go/chrome-feature-code-coverage-v2 and choose '365631664')

Low-Coverage-Reason: primarily glue code & very simple preference
Bug: 361404422
Change-Id: I9e7bf9bf45beb1cded6f05e151a7dfedca6f221c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5896353
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1361809}