DtlsTransportInternal is an internal interface that does DTLS.
The DtlsTransportChannel will inherit from this class instead of the
TransportChannelImpl so that the ICE-related methods can be separated.
Once the DtlsTransportChannl inherits from this class. It would be
renamed as DtlsTransport.
BUG=none
Review-Url: https://codereview.webrtc.org/2597053002
Cr-Commit-Position: refs/heads/master@{#15888}