adaptivedemux2: Check stream selected instead of state

adaptivedemux2: Check stream selected instead of state

When combining stream flows, ignore streams that
are not selected, instead of checking whether
the stream state has changed yet.

Fixes another issue with dashdemux2 where it fails to
change to the next period when playing content with
several video, audio and text streams, as with
Manifest_MultiPeriod_1080p.mpd when seeking to 730
just before the end of the first period.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2838>