metricsd: Persist the report to disk if an upload fails.

If the metrics server is unreachable, we may need to resend a metrics
report later. Instead of keeping the staged report in memory, save it to
disk to avoid loosing data if the system restarts or crashes.

BUG: 23033262
TEST: unit tests.

Change-Id: Idd14964e40f022952469f47d675d8cda9586d7cd