ART: Fix inefficent find/rfind

Use character instead of string versions where possible.

Bug: 32619234
Test: m test-art-host
Change-Id: Ifa0a68f17cf9fac5a8c15a6a1f1d085f0c8dcdd7
diff --git a/runtime/runtime_callbacks_test.cc b/runtime/runtime_callbacks_test.cc
index 983278d..a4d6c3d 100644
--- a/runtime/runtime_callbacks_test.cc
+++ b/runtime/runtime_callbacks_test.cc
@@ -261,7 +261,7 @@
       std::string location(initial_dex_file.GetLocation());
       std::string event =
           std::string("PreDefine:") + descriptor + " <" +
-          location.substr(location.rfind("/") + 1, location.size()) + ">";
+          location.substr(location.rfind('/') + 1, location.size()) + ">";
       data.push_back(event);
     }