Fix UAF in AndroidVideoTrackSource::SetState

Tasks posted to the signaling thread could execute after destruction
because the safety flag was not being marked as not alive. This caused
use-after-free crashes (SIGSEGV) in callbacks like FireOnChange.

This change ensures that the safety flag is marked as not alive in the
destructor, preventing tasks from running post-destruction.

Bug: b/403168866
Change-Id: I6ea1db070d32305725b8e7cd34d8b636f8ac70c9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462920
Reviewed-by: Per Åhgren <peah@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Viktor Grönroos <wilhelmsson@google.com>
Cr-Commit-Position: refs/heads/main@{#47456}