hlsdemux: When updating a non-live playlist make sure to find the current sequence...

hlsdemux: When updating a non-live playlist make sure to find the current sequence by time

Sequence numbers in different playlists are not guaranteed to be the same for the
same position, e.g. fragments could have different durations in different playlists.

In theory we should do exactly the same for live playlists, but unfortunately we can't
because doing this kind of seeking requires the complete playlist since we started
playback. For live playlists the server is however dropping fragments in the beginning
over time and we have no absolute time references.