📰 Add a notifications settings page for pre-O devices.

Bug: 746227
Change-Id: Icb5831b25c8c3a05625b6a286264307b95c96b5b
Reviewed-on: https://chromium-review.googlesource.com/589848
Commit-Queue: Peter Conn <peconn@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492596}