Fix save image/sound files to relative path in kvtml2writer

The soundUrl of a translation parley passed in is "file:///path/to/kvtml/kvtmlfilename_files/File:En-us-Word.ogg", and the m_doc->url() is "file:///path/to/kvtml/kvtmlfilename.kvtml". By the old logic saving as relative url, the result will be "file:///path/to/kvtml/file:///path/to/kvtml/kvtmlfilename_files/Files:En-us-word.ogg".

REVIEW: 129992