webrtc: fix the location of signalling-state change notification
1. The spec indicates that the notification should occur near the end of 'setting the description' processing 2. The current location with the drop of the lock could cause the 'check if negotiation is needed' logic to execute and become confused about the state of the webrtcbin's current local descriptions. In the bad case, the following assertions could be hit: g_assert (trans->mline < gst_sdp_message_medias_len (webrtc->current_local_description->sdp)); g_assert (trans->mline < gst_sdp_message_medias_len (webrtc->current_remote_description->sdp)); Moving the signalling state change later in the set description task means that checking for a renegotiation will early abort as the signalling state is not STABLE before the session description and transceivers have been updated.