Fix clang-tidy performance warnings in system/core.

* Use const reference parameter type to avoid unnecessary copy.
* Use more efficient overloaded string methods.

Bug: 30407689
Bug: 30411878
Test: build with WITH_TIDY=1
Change-Id: Ia5a00581e718d412255d6177e5a7c286cdfbec11
diff --git a/logcat/logcat.cpp b/logcat/logcat.cpp
index f64196f..b31db58 100644
--- a/logcat/logcat.cpp
+++ b/logcat/logcat.cpp
@@ -1256,7 +1256,7 @@
                         // example: "qemu_pipe,pipe:logcat"
                         // upon opening of /dev/qemu_pipe, the "pipe:logcat"
                         // string with trailing '\0' should be written to the fd
-                        size_t pos = devname.find(",");
+                        size_t pos = devname.find(',');
                         if (pos != std::string::npos) {
                             pipePurpose = devname.substr(pos + 1);
                             devname = devname.substr(0, pos);