Use specific users for metricsd/metrics_collector.

/data/misc/metricsd only needs to be accessed by metricsd.
/data/misc/metrics_collector only needs to be accessed by
metrics_collector.

/data/misc/metrics needs to be writable by metrics_collector
and readable by metricsd. Make it 0750 metrics_collector:system
and keep running metricsd as group 'system' to allow reading.

Bug: 26337609
Change-Id: I169573cc547f154daa8b65c5f13bdc20964a3f2c