[ios] Add Prefs to track new Most Visited Sites in top four

kIosLatestMostVisitedSites saved the latest fetched Most Visited Sites
so that the client can detect new sites in the top four.

kIosMagicStackSegmentationMVTImpressionsSinceFreshness will keep track
of the number of impressions since the last change to the top four
Most Visited Sites so that it can be sent to the Segmentation Platform
for ranking.

IOSMostVisitedTopSitesChanged user action is also logged when there is
a new top four site as a way for the Segmentation team to investigate
a future where all of this can be handled by aggregating metrics.

Bug: 1462664
Change-Id: I0bb35d938567f3f11e4da63c63dde44e3779a95b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4682385
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1175598}