metrics: Listen to session manager for screen lock/unlock signals.

This should fix a pretty broken DailyUseTime metric. Also, use
contstants from service_constants.h where available.

BUG=chromium:216382,chromium:234799
TEST=unit tests; stop metrics_daemon on the device, then ran metrics_daemon
--nodaemon and inspected console log output while logging in/out as well as on
suspend/resume.
CQ-DEPEND=Ibfcc54c8dbf145cccd5ef871c7c9701b8312eb9e

Change-Id: I69ca0ea9594a496453f0933147ec66b0fa334718
Reviewed-on: https://gerrit.chromium.org/gerrit/49468


Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Darin Petkov <petkov@chromium.org>