update_engine: Use org.chromium.NetworkProxyService.

Make update_engine call Chrome's new
org.chromium.NetworkProxyService D-Bus service to resolve
network proxies instead of using
org.chromium.LibCrosService. The new service supports
asynchronous replies instead of responding via D-Bus
signals.

BUG=chromium:446115,chromium:703217
TEST=unit tests pass; also added debug logging and verified
     that chrome's proxy settings are used

Change-Id: Iebd268ea3e551c0760416d955828b9d7ebf851fb
Reviewed-on: https://chromium-review.googlesource.com/497491


Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>