Move arc notification code to ash/public/cpp/external_arc

* Changed the ArcNotificationManager so that it can be injected
  to MessageCenterController instead of creating it inside
* Moved arc notification impl to ash/public/cpp/external_arc/message_center
* Moved arc notification interfaces and constants to
   ash/public/cpp/message_center

AX-Relnotes: n/a.
Bug: 1090616
Change-Id: I995fc5749ab0fbe18d43b451e6553b8d82728e5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2244656
Commit-Queue: Mitsuru Oshima (slow:gardening) <oshima@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#778970}