Use const auto&/auto&& in adb.

Change-Id: I74a7e511302e15e207906f572d181634e0ed5604
diff --git a/adb/file_sync_service.cpp b/adb/file_sync_service.cpp
index 9b56392..298ed82 100644
--- a/adb/file_sync_service.cpp
+++ b/adb/file_sync_service.cpp
@@ -56,7 +56,7 @@
     path_components.pop_back(); // For "/system/bin/sh", only create "/system/bin".
 
     std::string partial_path;
-    for (auto& path_component : path_components) {
+    for (const auto& path_component : path_components) {
         if (partial_path.back() != OS_PATH_SEPARATOR) partial_path += OS_PATH_SEPARATOR;
         partial_path += path_component;