rtsp-stream: Ensure that the pipeline is live and later-added udpsrcs are syncing...

rtsp-stream: Ensure that the pipeline is live and later-added udpsrcs are syncing the state with the parent bin

Without this, RECORD pipelines are broken because
a) we wait for ASYNC_DONE which never happens anymore because udpsrc would be
added later. Previously it was there earlier and due to NO_PREROLL caused the
pipeline to preroll immediately
b) the udpsrc for the pipeline is added later and never set to PLAYING state,
as the corresponding code previously was only for PLAY pipelines.

https://bugzilla.gnome.org/show_bug.cgi?id=763281