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
metricsd should report the stateful partition's usage, both in MB and as
a percentage of the available space.
This measure will be reported every 15 minutes.
BUG: 24132871
Change-Id: I1cd8d651a980f90a57be9cc2a5baf6ed59794470