Replace StringFromInt with std::to_string

Updated version of #47. Android should support to_string now that
we use a modern version of libc++ when building.

Source/Core/Common/StringUtil.cpp

@@ -283,13 +283,6 @@ bool TryParse(const std::string& str, bool* const output)
   return true;
 }
 
-std::string StringFromInt(int value)
-{
-  char temp[16];
-  sprintf(temp, "%i", value);
-  return temp;
-}
-
 std::string StringFromBool(bool value)
 {
   return value ? "True" : "False";