Create pref for glanceables client registration

Define a 'kGlanceablesEnabled' pref to control the visibility of the
glanceables UI.
The pref will be set to default to the state of the feature
'GlanceablesV2'
Before registering a new client for the glanceables service, verify that
'kGlanceablesEnabled' pref is true.

Add a pref observer to (un)register the client as soon as the pref is
toggled.

Bug: b:283371147
Change-Id: Ic4bf932b26281a274394a30960e9d3322805b6ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4630270
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1161566}