[Extensions c2s] Populate user permissions in manager from pref

User permissions are persisted in `kUserPermissions` pref.
PermissionsManager reads from `kUserPermissions` and stores a local copy
for easy access and modification.

Bug: 1268198
Change-Id: I1fb9bfe56c07577de65e0cecdca4ad449c5a16a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3371161


Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#959818}