Fix bugprone-macro-parentheses clang-tidy issues

This CL also fixes a performance-unnecessary-copy-initialization issue which was present on the master branch and prevented m tidy-art to complete successfully.

Test: m tidy-art
Bug: 213953102
Change-Id: I821d797be4a866d9e1546e62bafa5a8bb0ac0578
diff --git a/libnativeloader/native_loader_test.cpp b/libnativeloader/native_loader_test.cpp
index 9648713..7663f90 100644
--- a/libnativeloader/native_loader_test.cpp
+++ b/libnativeloader/native_loader_test.cpp
@@ -169,7 +169,7 @@
 
 std::string default_public_and_extended_libraries() {
   std::string public_libs = default_public_libraries();
-  std::string ext_libs = extended_public_libraries();
+  const std::string& ext_libs = extended_public_libraries();
   if (!ext_libs.empty()) {
     public_libs = public_libs + ":" + ext_libs;
   }