ot::Spinel::Encoder::OverwriteWithLastStatusError() method

Overwrites the property key with `LAST_STATUS` in a property update command frame. Should be only used after a successful `BeginFrame(aHeader, aCommand, aPropertyKey)`, otherwise, its behavior is undefined. Moves the write position back to saved position by `BeginFrame()` and replaces the property key `SPINEL_PROP_LAST_STATUS` and writes the given spinel status error.