thermalservice: run as user+group system, not root

Bug: 66954030
Test: manual on taimen with artificially low thermal thresholds
Change-Id: I2a9275806b4cb710d115ba4cc4e4f161eefd1839
diff --git a/services/thermalservice/thermalservice.rc b/services/thermalservice/thermalservice.rc
index b9836ce..94c2c78 100644
--- a/services/thermalservice/thermalservice.rc
+++ b/services/thermalservice/thermalservice.rc
@@ -1,2 +1,4 @@
 service thermalservice /system/bin/thermalserviced
     class core
+    user system
+    group system