The factory follows the same principles as PeerConnectionFactory;
various modules can be passed into its constructor but default
implementations are provided. Currently the only object the factory can
create is a UdpTransport (need to start somewhere).
UdpTransportChannel (renamed to UdpTransport)
will now accept a socket passed into its constructor,
relying on the factory to create the socket. This allows some
simplifications to be made, such as getting rid of "State" since the
only states are now "has destination set or doesn't".
BUG=webrtc:7013
Review-Url: https://codereview.webrtc.org/2632613002
Cr-Commit-Position: refs/heads/master@{#16154}
