webrtcbin: Don't consider RTP receivers stopped

We don't support stopping RTP receivers currently so let's not consider them all stopped all the time. This fixes some of the ICE/DTLS state change handling and specifically fixes the ICE gathering state.

Previously the ICE gathering state was immediately going from NEW to COMPLETE because it considered all transceivers stopped and as such all activate transceivers were finished gathering ICE candidates.

Fixes #1126 (closed)