init.rc: add healthd to system group to allow write to /dev/cpuset

Also add to root group to avoid introducting new bugs.

Bug: 22699101
Change-Id: I9da31e0cc955efd711df3f4c6b17e39d74c01549
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 7af2b77..b7e238a 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -532,6 +532,7 @@
     class core
     critical
     seclabel u:r:healthd:s0
+    group root system
 
 service console /system/bin/sh
     class core