Update DTLS transport lookup methods in controller, pc, sdp

Make sure that methods in JsepTransportController are consistent with
the pattern there with blocking calls from the signaling thread to the
network thread. This consistency is to aid with future work of removing
or avoiding these blocking calls.

Key changes:
* Rename the existing network-thread-bound method in
  JsepTransportController to LookupDtlsTransportByMid_n.
* Introduce a new LookupDtlsTransportByMid that performs the
  BlockingCall to the network thread.
* Remove PeerConnection::LookupDtlsTransportByMidInternal.
* Remove the static helper LookupDtlsTransportByMid in
  sdp_offer_answer.cc.
* Update tests.

Bug: webrtc:9987
Change-Id: I64b7eb9b7f368dcd21b87a177b700a2e811f8c73
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/429520
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46359}