system_api: Add DBus constants and protos for OOBE config BUG=chromium:869209 TEST=emerges Change-Id: I73228de51221000312801719d8c9e77238da654b Reviewed-on: https://chromium-review.googlesource.com/1197211 Commit-Ready: Zentaro Kavanagh <[email protected]> Tested-by: Zentaro Kavanagh <[email protected]> Reviewed-by: Zentaro Kavanagh <[email protected]>