metricsd: Only collect metrics over a short period.

Instead of reporting the metrics over both a long and a short period,
collect and report only over a short period. This makes the code simpler
and the metrics easier to understand.

Also move the collection out of metrics_daemon and into a separate
collector to make it simpler to understand.

BUG: 24464945

Change-Id: I17e52536aaa75321a5e34f42ed488545c2c3efde