The new streams should not be exposed until all streams are done with the
current fragment. The old code is incorrect and actually only checked the
current stream. Fix this by properly checking all streams.

Also, ignore the current stream. The code is only reached when the current
stream finished downloading and since
07f49f15 ("adaptivedemux: On EOS, handle it
before waking download loop") download_finished is set after
gst_adaptive_demux_stream_advance_fragment_unlocked() is called.

Without this HLS playback with multiple streams is broken, because the new
streams are never exposed.

https://bugzilla.gnome.org/show_bug.cgi?id=770075