Use const refs instead of copying strings.

Change-Id: I7513d4d8991b637c86290eb0b2e6e73caff955a4
diff --git a/test/cts/utils.cc b/test/cts/utils.cc
index b11cddf..c61eff7 100644
--- a/test/cts/utils.cc
+++ b/test/cts/utils.cc
@@ -124,7 +124,7 @@
   });
 }
 
-void StopApp(std::string app_name) {
+void StopApp(const std::string& app_name) {
   std::string stop_cmd = "am force-stop " + app_name;
   system(stop_cmd.c_str());
 }