[PosixEventEngine] Ensure threads are shut down before destroying other engine state (#31152)

* Add check for engine life on shutdown

This fixes a race condition where closures attempt to deregister themselves after the engine has been destroyed

* start initialized

* ensure threads are shut down before other engine state is destroyed

src/core/lib/event_engine/posix_engine/posix_engine.h

@@ -107,12 +107,11 @@ class PosixEventEngine final : public EventEngine {
   EventEngine::TaskHandle RunAfterInternal(Duration when,
                                            absl::AnyInvocable<void()> cb);
 
-  posix_engine::TimerManager timer_manager_;
-  ThreadedExecutor executor_{2};
-
   grpc_core::Mutex mu_;
   TaskHandleSet known_handles_ ABSL_GUARDED_BY(mu_);
   std::atomic<intptr_t> aba_token_{0};
+  posix_engine::TimerManager timer_manager_;
+  ThreadedExecutor executor_{2};
 };
 
 }  // namespace experimental