Don't use AtomicFreeList to squash TSAN complains

The previous fix attempt didn't work on some test. There are no known
bugs with AtomicFreeList other than tripping TSAN, and it's unclear
that TSAN isn't at fault.

However, switching to using a mutex works fine and doesn't appear to
be slower (it's actually faster with synthetic benchmarks on macOS)

BUGS=[377369108]

libs/utils/include/utils/JobSystem.h

@@ -446,7 +446,7 @@ class JobSystem {
     Condition mWaiterCondition;
 
     std::atomic<int32_t> mActiveJobs = { 0 };
-    utils::Arena<utils::ThreadSafeObjectPoolAllocator<Job>, LockingPolicy::NoLock> mJobPool;
+    utils::Arena<utils::ObjectPoolAllocator<Job>, LockingPolicy::Mutex> mJobPool;
 
     template <typename T>
     using aligned_vector = std::vector<T, utils::STLAlignedAllocator<T>>;