ControllerInterface: Don't pass m_devices to the backends

Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.

Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -121,6 +121,7 @@ public:
   void Initialize(void* const hwnd);
   void Reinitialize();
   void Shutdown();
+  void AddDevice(ciface::Core::Device* device);
   bool IsInit() const { return m_is_init; }
   void UpdateReference(ControlReference* control,
                        const ciface::Core::DeviceQualifier& default_device) const;