Move CallbackList member variables in JsepTransportController

Move these callback variables to the member variable section in the
class and below other state.

Bug: none
Change-Id: Ia3b7ac9b99a853ed2ce959bd8d65933b9ceae1a6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/426700
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46224}