AbstractSession: Only start encodedStream if it's ready

AbstractSession::requestStreamingEnable can be called before the session was able to configure encodedStream. In that case the stream would be created with wrong encoder settings and the streaming fails. Wait for setStarted(true) to be called first.

This case was easy to trigger if the connection attempt triggered a portal permission dialog. After accepting the request, krdpserver attempted to stream using libvpx.