Fix selected challenge option setting in VCN 3CSC challenge option

`selected_idv_challenge_option` is the higher-level dictionary that
contains the specific challenge options, so this CL fixes the VCN 3CSC
case to set `cvc_challenge_option` inside of
`selected_idv_challenge_option` instead of directly in the request
content.

Fixed: 356665737
Change-Id: Iac43ce27ec35a149f1074c4761a92866d6b6aac9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825190


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Sujie Zhu <sujiezhu@google.com>
Commit-Queue: Vinny Persky <vinnypersky@google.com>
Cr-Commit-Position: refs/heads/main@{#1349386}