Shortcuts: Add policy observer

* Add policy observer in accelerator_prefs, both controller and
  provider can listen to it, any change in prefs will first notify the
  prefs class then to the other two classes.
* Create accelerator_prefs_delegate between ASH and browser to ensure
  that we only check the pref if the user is managed

Bug: b/216049298
Test: ash_unittest
Change-Id: I753b1db2d242df606a43c9beefb6db87a4c29686
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4777996
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Longbo Wei <longbowei@google.com>
Cr-Commit-Position: refs/heads/main@{#1191490}