Ensure iomgr does not track Forkable threads (#30572)

* Ensure iomgr does not track Forkable threads

New uses of grpc_core::Thread should not be tracked by iomgr. It would
be nice to enforce this, but that can come later. This should unblock
python releases.

* tell parent & child that forking is complete

src/core/lib/event_engine/posix_engine/timer_manager.cc

@@ -62,8 +62,9 @@ void TimerManager::StartThread() {
   ++thread_count_;
   auto* thread = new RunThreadArgs();
   thread->self = this;
-  thread->thread =
-      grpc_core::Thread("timer_manager", &TimerManager::RunThread, thread);
+  thread->thread = grpc_core::Thread(
+      "timer_manager", &TimerManager::RunThread, thread, nullptr,
+      grpc_core::Thread::Options().set_tracked(false));
   thread->thread.Start();
 }
 
@@ -273,6 +274,7 @@ void TimerManager::PostforkParent() {
     StartThread();
   }
   prefork_thread_count_ = 0;
+  forking_ = false;
 }
 
 void TimerManager::PostforkChild() {
@@ -281,6 +283,7 @@ void TimerManager::PostforkChild() {
     StartThread();
   }
   prefork_thread_count_ = 0;
+  forking_ = false;
 }
 
 }  // namespace posix_engine

src/core/lib/event_engine/thread_pool.cc

@@ -32,7 +32,7 @@ ThreadPool::Thread::Thread(ThreadPool* pool)
       thd_(
           "posix_eventengine_pool",
           [](void* th) { static_cast<ThreadPool::Thread*>(th)->ThreadFunc(); },
-          this) {
+          this, nullptr, grpc_core::Thread::Options().set_tracked(false)) {
   thd_.Start();
 }
 ThreadPool::Thread::~Thread() { thd_.Join(); }