Move SetWritable and SetDestination calls after setting the state+role

This is to avoid a potential race triggered by in downstream code from
FakeDtlsTransport.

Bug: none
Change-Id: I1be62b2b46fd3ac7b08ee1cc5d5d10f9a45fd3e6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/428960
Reviewed-by: Tony Herre <herre@google.com>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#46328}