kmswebrtcendpoint: use g_clear_object for destroying transport

Change-Id: I01b4b3acc8cb04890953e69bea3bc3e28aed227d
kmswebrtcendpoint: fix DTLS negotiation with sendonly and recvonly

We need to add dtlssrtpenc elements to allow DTLS negotiation.

Change-Id: I9ba7260696aa0b8b2624651de9ebc68cb603e259
webrtcendpoint: add test for vp8 sendonly and recvonly

Change-Id: I92a377db856dcd61aad121c340dc5cc259c5d153