ART: Fix tidy warnings

Switch to char versions of find variants.

Add "explicit" constructor variants or refactor and
remove defaults.

Use const references.

Bug: 32619234
Test: m test-art-host
Change-Id: I970cc2f47d6cf8f0c74104b994b075b2fafb3d45
diff --git a/patchoat/patchoat.cc b/patchoat/patchoat.cc
index 7ea5bea..3c8c1a3 100644
--- a/patchoat/patchoat.cc
+++ b/patchoat/patchoat.cc
@@ -1068,7 +1068,7 @@
   TimingLogger::ScopedTiming pt("patch image and oat", &timings);
 
   std::string output_directory =
-      output_image_filename.substr(0, output_image_filename.find_last_of("/"));
+      output_image_filename.substr(0, output_image_filename.find_last_of('/'));
   bool ret = PatchOat::Patch(input_image_location, base_delta, output_directory, isa, &timings);
 
   if (kIsDebugBuild) {