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/data/etc/platform.xml b/data/etc/platform.xml
index d2c855b..f169f22 100644
--- a/data/etc/platform.xml
+++ b/data/etc/platform.xml
@@ -168,6 +168,8 @@
     <assign-permission name="android.permission.ACCESS_LOWPAN_STATE" uid="lowpan" />
     <assign-permission name="android.permission.MANAGE_LOWPAN_INTERFACES" uid="lowpan" />
 
+    <assign-permission name="android.permission.STATSCOMPANION" uid="statsd" />
+
     <!-- This is a list of all the libraries available for application
          code to link against. -->