Fix clang-tidy performance warnings in frameworks/rs.

* Use more efficient overloaded string methods.

Bug: 30411878
Test: build with WITH_TIDY=1
Change-Id: I030d50ecce8d4b689181718c48be7e9f75189c36
diff --git a/rsScriptC.cpp b/rsScriptC.cpp
index 669954a..a0dcad3 100644
--- a/rsScriptC.cpp
+++ b/rsScriptC.cpp
@@ -79,7 +79,7 @@
          * The character at currPos should be a path separator.  We need to look
          * for the next one.
          */
-        int nextPos = cacheDirString.find(OS_PATH_SEPARATOR_STR, currPos + 1);
+        int nextPos = cacheDirString.find(OS_PATH_SEPARATOR, currPos + 1);
 
         if (nextPos > 0) {
             // A new path separator has been found.