metrics: Clean up user-active code.

Stop listening for the soon-to-deleted PowerStateChanged
D-Bus signal from powerd; the code that was listening for
it had some issues (there was no guarantee that it'd run
before the system suspended, and resumes weren't handled at
all).

Also remove the session-state- and screen-lock-handling
code; it provided an inaccurate view of user activity (what
if the user is listening to music with the screen locked or
if they don't have screen-locking enabled?).

Track uptime instead via a timer that fires every five
minutes.

BUG=chromium:359619
TEST=manual: ran metrics_daemon --nodaemon and watched the
     logs

Change-Id: I4ad74773daefa01afdea080d20001ff1944c2eee
Reviewed-on: https://chromium-review.googlesource.com/195491


Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>