Move wstring to string conversion to StringUtils.cpp and update most HID get serial number functions to use it

Controllers/MountainKeyboardController/MountainKeyboardController.cpp

@@ -13,6 +13,7 @@
 #include <cstring>
 #include <thread>
 #include "MountainKeyboardController.h"
+#include "StringUtils.h"
 
 using namespace std::chrono_literals;
 
@@ -42,10 +43,7 @@ std::string MountainKeyboardController::GetSerialString()
         return("");
     }
 
-    std::wstring return_wstring = serial_string;
-    std::string return_string(return_wstring.begin(), return_wstring.end());
-
-    return(return_string);
+    return(StringUtils::wstring_to_string(serial_string));
 }
 
 void MountainKeyboardController::SelectMode(unsigned char mode_idx)