Make video_options_ const in SdpOfferAnswerHandler

Initialize `video_options_` directly in the constructor initializer list
and declare it as a const member variable. This improves safety and
thread-safety by avoiding post-construction modifications and removing
the thread guard since const members are thread-safe to read.

Bug: none
Change-Id: I81a9020de72ca0332cd09410ce9f04d796282ca3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476740
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47853}