buffet: Grant buffet group access to buffet DBus API. BUG=chromium:434766 TEST=manual Change-Id: I28bad564fdc1da04b3e80b2e622904c08ef0172d Reviewed-on: https://chromium-review.googlesource.com/230851 Tested-by: Anton Muhin <antonm@chromium.org> Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org>