buffet: Add State property to Manager BUG=chromium:356368 TEST=Modified buffet_BasicDBusAPI verifies that Get and GetAll work. Change-Id: I3af7cbbdee6d47405786420b0d00a71b0f883ec7 Reviewed-on: https://chromium-review.googlesource.com/192002 Tested-by: Christopher Wiley <wiley@chromium.org> Reviewed-by: Chris Sosa <sosa@chromium.org> Commit-Queue: Christopher Wiley <wiley@chromium.org>