dashdemux2: Avoid pushing stream-start events after seeking

After seeks and when not downloading headers, we should not push a new stream_start as it leads to a total reset of the demuxer which then requires new headers so it knows the stream it is working with. Pushing the stream-start after a seek (right after prerolling in that particular case) leads to:

<qtdemux0> error: This file contains no playable streams.
<qtdemux0> error: no known streams found