Camera does not update active property when device is disconnected

Observed on macOS and iOS. When using an external USB camera, if you activate the QCamera and then unplug the camera, the 'active' property is not updated to reflect that we are no longer sending.

The same applies to the 'isAvailable' property, which should tell us whether the camera is ready to go active or not. In the case of a disconnection, it should be set to false.

The same also applies to cameras that are still connected, but go into 'AVCaptureDevice.isSuspended'. This commonly happens if using a Macbook integrated camera, and closing the lid.

This already works correctly on Android, so it's useful for Qt to ensure the same behavior on both platforms.