Pass SettingsHandler buffers by const ref instead of rvalue ref (since the contents are copied either way)

[nlebeck]

SettingsHandler::Buffer is a type alias to a std::array of u8. Moving a std::array of primitives doesn't actually avoid any copies, since the elements are stored inline (https://stackoverflow.com/a/28988913). Changing these methods to take a const ref instead of an rvalue ref makes it clear that we are actually copying the arrays.