Migrate engagement to OnceCallback

This removes 'use_once_callback = false' flag from BUILD.gn from
cr/br/engagement, and fixes all implementations of the interface.

Tbr: tommycli@chromium.org
Bug: 714018
Change-Id: Ic139ba25eb020caa384735292ee62ddfb1272ba9
Reviewed-on: https://chromium-review.googlesource.com/1002220
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550529}
5 files changed