libcamera: disconnect camera in spa_libcamera_stream_off()

The camera is connected in the spa_libcamera_stream_on() function but the
disconnect happen in LibCamera::stop(). It makes more sense to have this
two operations separated as it's done for the connection and start, since
the camera then can be stopped/started without needing to do a reconnect.

Move the camera disconnect to spa_libcamera_stream_off() for the reason
mentioned above but also to make the code more consistent and symmetric.