threadshare: make live sources return NoPreroll from PlayingToPaused
Not returning NoPreroll in that transition causes downstream sinks to wait for preroll forever.
Reproduce with:
gst-launch-1.0 audiotestsrc ! udpsink host=127.0.0.1 port=50000 gst-launch-1.0 ts-udpsrc address=127.0.0.1 port=50000 ! fakesink
ctrl + C in the receiver pipeline -> hangs forever