video_capture: move capture_checker check after StopCapture in V4L2 dtor

Before: The V4L2 capture destructor ran
`RTC_CHECK_RUNS_SERIALIZED(&capture_checker_)` before `StopCapture()`,
which can assert while the capture thread is still active.
After: The destructor now calls `StopCapture()` first, then validates
serialized access, with an explanatory comment about the capture
thread lifecycle.

Bug: webrtc:479850459
Change-Id: I147b55bf0f1592586290dcdeba6efc3f1025406a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444800
Reviewed-by: Jan Grulich <grulja@gmail.com>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Jan Grulich <grulja@gmail.com>
Cr-Commit-Position: refs/heads/main@{#46866}