metricsd: Use different directories for each daemon.

Instead of using a single directory for both the internal data of
metricsd and metrics_collector and the shared files (metrics samples log
file and the metrics enabled file), we should use separate directory to
allow for a finer access control.

The new structure will be:
* /data/misc/metrics for the files accessible to all daemons reporting
  metrics, metricsd and metrics_collector.
* /data/misc/metricsd for the private files of metricsd.
* /data/misc/metrics_collector for the private files of
  metrics_collector.

Bug: 25886951
Test: Unit tests.
Test: Manual: metricsd and metrics_collector run without errors.

Change-Id: I006d19f45f5f419d2b08744126c2e2a0b899c9fa