[extractor/common] Fix format_id when `last_media` is None and always include `m3u8_i...

[extractor/common] Fix format_id when `last_media` is None and always include `m3u8_id` if present

The rationale behind `m3u8_id` was to resolve duplicates when processing several m3u8 playlists within the same media that give equal resulting `format_id`'s,
e.g. `youtube-dl http://www.rts.ch/play/tv/passe-moi-les-jumelles/video/la-fee-des-bois-mustang-les-chemins-du-vent?id=3854925 -F`