demux: playlist: fix metadata issue during live streams from m3u file

The metadata priority for individual tracks in the m3u file is stored to indicate whether the file contains valid metadata for a given track. The priority is set to VLC_META_PRIORITY_PLAYLIST if valid metadata is present, and VLC_META_PRIORITY_BASIC if it is not.

If a track contains valid metadata (defined by the #EXTINF or #EXTALBUMARTURL directives), it will not be overwritten by the demuxer. Otherwise, the metadata may be updated by the demuxer.