metricsd: Persist metrics on shutdown.

When metricsd receives a SIGTERM signal (on system shutdown for
example), save the metrics to disk before exiting.

Bug: 26184202
Test: Start metricsd. Log a metric sample. Stop metricsd. The metric is
saved to disk.

Change-Id: Ib224efa8837130393fac5d06fa80d320263cfc84