hle: kernel: Refactor to allocate a ServiceThread per service handler.

- Previously, we would allocate a thread per session, which adds new threads on CloneCurrentObject.
- This results in race conditions with N sessions queuing requests to the same service interface.
- Fixes Pokken Tournament DX crashes/softlocks, which were regressed by #6347.

src/core/hle/service/ns/pl_u.cpp

@@ -254,8 +254,6 @@ void PL_U::GetSharedMemoryNativeHandle(Kernel::HLERequestContext& ctx) {
     LOG_DEBUG(Service_NS, "called");
 
     // Create shared font memory object
-    auto& kernel = system.Kernel();
-
     std::memcpy(kernel.GetFontSharedMem().GetPointer(), impl->shared_font->data(),
                 impl->shared_font->size());