ice_transport: avoid dirty underlyingIce()

Let the IceTransport layer stopped it's current read if destroyed
(as it uses a peerChannel, this channel should be cancelled to
let upper layer stop correctly all operations).
Call the shutdown callback whenever the ICE is really destroyed
(so caller must not re-use the ICE pointer).
Finally, remove some dirty underlyingIce() accesses in peer_connection

Change-Id: Icca73d9af273297a558121ffddc991ac6617ece2
GitLab: #703