metricsd: Don't cache the metrics status in the daemon.

metrics_daemon should never get a stale answer on whether or not the
metrics are enabled.
This is important as metrics_daemon will be the "source of truth" for
other components.

BUG: 24386281
TEST: unit tests.

Change-Id: I573568abe5d1b840683cede2fdf32cdae028a81a