[Bookmarks Bar] Use a SharedPref for tablets instead of syncing UserPref

Contributor: Jenna Himawan, jhimawan@google.com

This CL updates the way that the Bookmark Bar on Clank handles syncing
preferences.

Before this update, the user preference for showing the bookmark bar
always matched that of Desktop. With this change, the bookmark bar pref
will be split between Desktop and tablets. For Desktop we will sync
with the user's existing bookmark bar preference (the previous impl).
For tablets, we will not sync with that user pref and will instead use
a local device preference. With this CL we add logic to allow for
these two parallel approaches. We also include a new FeatureParam so
that we can set different default local prefs on tablets during
experimentation. We update the keyboard shortcut impl, settings,
observers, tests, etc to all match this new implementation.

Bug: 440359932
Change-Id: Ia659f3c764aff0ba20f965b8a793458a36ae32ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6880777
Reviewed-by: Brandon Wylie <wylieb@google.com>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Jenna Himawan <jhimawan@google.com>
Cr-Commit-Position: refs/heads/main@{#1509714}