Moved JoystickId from UnityInputDevice to InputDevice

Moved the JoystickId property from UnityInputDevice to InputDevice. I did this so that when doing things like the code below, I can use the JoystickId. This just seems to make it easy to track input devices especially when attached and detached as it gives them a somewhat unique identifier to use. This could be helpful if I want a player to always be using a specific input device and if it disconnected and reconnects, I can easily handle this.

```csharp
foreach (InputDevice inputDevice in InputManager.Devices) {
    Debug.Log("Checking input with id: " + inputDevice.JoystickId);
}
```