metricsd: Persist the metrics to disk periodically.

Every now and then (5 minutes by default), the uploader will persist the
current metrics to disk to avoid losing them in case we exit
unexpectedly (reboot or crash).
When starting up, metricsd will load the previously saved log and resume
the metrics collection from there.

Bug: 25670584
Test: Unit tests.
Test: manual: restart metricsd. The saved log is detected and parsed
correctly.
Test: manual: Send a sample to metricsd, send SIGTERM to metricsd, the
log is saved to disk, metricsd restarts and picks up the log where it
left.

Change-Id: I4cefc62c7ea1fa51333d84d8a7ba0a2e9c7fd58f