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/cmdline/cmdline.h b/cmdline/cmdline.h
index dec9c83..6e042c3 100644
--- a/cmdline/cmdline.h
+++ b/cmdline/cmdline.h
@@ -234,7 +234,7 @@
     // Checks for --boot-image location.
     {
       std::string boot_image_location = boot_image_location_;
-      size_t file_name_idx = boot_image_location.rfind("/");
+      size_t file_name_idx = boot_image_location.rfind('/');
       if (file_name_idx == std::string::npos) {  // Prevent a InsertIsaDirectory check failure.
         *error_msg = "Boot image location must have a / in it";
         return false;
@@ -244,7 +244,7 @@
       // This prevents a common error "Could not create an image space..." when initing the Runtime.
       if (file_name_idx != std::string::npos) {
         std::string no_file_name = boot_image_location.substr(0, file_name_idx);
-        size_t ancestor_dirs_idx = no_file_name.rfind("/");
+        size_t ancestor_dirs_idx = no_file_name.rfind('/');
 
         std::string parent_dir_name;
         if (ancestor_dirs_idx != std::string::npos) {