Consolidate RtpTransport members in JsepTransport.

Merges unencrypted_rtp_transport_ and dtls_srtp_transport_ into a
single rtp_transport_ member of type std::unique_ptr<RtpTransport>.
Updates JsepTransport constructor to accept a single transport.
Updates JsepTransportController to instantiate the appropriate transport
(encrypted or unencrypted) and pass it.

This simplifies JsepTransport by removing mutually exclusive members and
aligning with the polymorphic RtpTransport interface.

Bug: webrtc:360058654
Change-Id: If7ce6099b4b228d0c07fb0064c1843afbe7b7ea8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/429040
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46341}