Persist active metrics to disk and read back

For metric activation that spans across boots, we need to persist active
metrics onto disk upon shutdown and load them on boot.

Bug: 123904359
Test: unit test
Change-Id: I5a4142a42595c8c132175fb574c3aa2ad30dcac0
diff --git a/cmds/statsd/statsd.rc b/cmds/statsd/statsd.rc
index cbf2a8d..e0cbd5d 100644
--- a/cmds/statsd/statsd.rc
+++ b/cmds/statsd/statsd.rc
@@ -26,3 +26,4 @@
     # Create directory for statsd
     mkdir /data/misc/stats-data/ 0770 statsd system
     mkdir /data/misc/stats-service/ 0770 statsd system
+    mkdir /data/misc/stats-active-metric/ 0770 statsd system