video_capture: fix stale best metrics in capability selection

Before: When preferring a specific video type, the best width/height/fps were not updated, so later comparisons used stale best metrics and could overwrite the preferred format even with identical fps.
After: Update best width/height/fps alongside the preferred format selection and avoid overwriting with same-fps exact matches, making selection stable and consistent.
Bug: webrtc:42226038
Change-Id: I8133e68e09c7687782533515708b0191b7d5084a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442500
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46708}