Fix clang-tidy performance warnings in syste/core.

* Use const reference type for parameters to avoid unnecessary copy.
* Suppress warning of not using faster overloaded string find function.

Bug: 30407689
Bug: 30411878
Change-Id: Ie79dbe21899867bc62031f8618bb1322b8071525
Test: build with WITH_TIDY=1
diff --git a/fastboot/socket_test.cpp b/fastboot/socket_test.cpp
index affbdfd..373abc3 100644
--- a/fastboot/socket_test.cpp
+++ b/fastboot/socket_test.cpp
@@ -34,7 +34,7 @@
 // Creates connected sockets |server| and |client|. Returns true on success.
 bool MakeConnectedSockets(Socket::Protocol protocol, std::unique_ptr<Socket>* server,
                           std::unique_ptr<Socket>* client,
-                          const std::string hostname = "localhost") {
+                          const std::string& hostname = "localhost") {
     *server = Socket::NewServer(protocol, 0);
     if (*server == nullptr) {
         ADD_FAILURE() << "Failed to create server.";