Fix clang-tidy performance warnings in aapt2.

* 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: Iaa0a6f18f64cf573bee49dd1466281ea4d37b410
diff --git a/tools/aapt2/LoadedApk.cpp b/tools/aapt2/LoadedApk.cpp
index 7e5efa1..abc0e4c 100644
--- a/tools/aapt2/LoadedApk.cpp
+++ b/tools/aapt2/LoadedApk.cpp
@@ -84,7 +84,7 @@
 
     std::string path = file->GetSource().path;
     // The name of the path has the format "<zip-file-name>@<path-to-file>".
-    path = path.substr(path.find("@") + 1);
+    path = path.substr(path.find('@') + 1);
 
     // Skip resources that are not referenced if requested.
     if (path.find("res/") == 0 && referenced_resources.find(path) == referenced_resources.end()) {