Make audio_options_ const in SdpOfferAnswerHandler

Initialize `audio_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: Id14b0defc05a0fbc469ec23241dbe1114d5ee2cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/476720
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@{#47850}