make statsd run in its own uid

+ Assign permission android.permission.STATSCOMPANION to statsd
+ Fixes in StatsCompanionService to allow statsd to get uid mapping

Test: manual
Change-Id: I3e6ca79eefed7f93a4588578c156321c4c278fd3
diff --git a/cmds/statsd/statsd.rc b/cmds/statsd/statsd.rc
index 9fb1027..437f9a8 100644
--- a/cmds/statsd/statsd.rc
+++ b/cmds/statsd/statsd.rc
@@ -14,5 +14,5 @@
 
 service statsd /system/bin/statsd
     class main
-    user system
-    group system
+    user statsd
+    group statsd log