Fix incorrect OwnedFactoryAndThreads teardown sequence

Reorder declaration of members in OwnedFactoryAndThreads so that env_
and signaling_thread_ are destroyed last. This ensures worker and
network threads can safely run teardown tasks (such as releasing proxy
objects) that marshal calls to the signaling thread or use environment
resources without accessing deleted objects.

Also, add a unit test that verifies the correct teardown order and
prevents regression.

Bug: b/515407473
Change-Id: I4f517cc99bf1d2ed05bce5f9de9a1d146fe0c154
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/479200
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47931}