Fix performance-for-range-copy warnings

Bug: 30413223
Test: make with WITH_TIDY=1 DEFAULT_GLOBAL_TIDY_CHECKS=-*,performance*
Change-Id: Ie481e88025a7a1f3abde8ff63420d5ccd8577e52
diff --git a/tools/aapt2/util/Files.cpp b/tools/aapt2/util/Files.cpp
index 5a8ff09..a407c22 100644
--- a/tools/aapt2/util/Files.cpp
+++ b/tools/aapt2/util/Files.cpp
@@ -165,7 +165,7 @@
 
 std::string PackageToPath(const StringPiece& package) {
   std::string out_path;
-  for (StringPiece part : util::Tokenize(package, '.')) {
+  for (const StringPiece& part : util::Tokenize(package, '.')) {
     AppendPath(&out_path, part);
   }
   return out_path;