AVFoundation: fix microphone permission when using the camera.

AVFoundation: fix microphone permission when using the camera.

The microphone permission was always requested when using the camera,
even when not actually using the microphone, which can only happen when
recording a video. The permission request is triggered by adding an
audio AVCaptureDeviceInput to the AVCaptureSession, which was done when
setting the camera to LoadedState. This is now done when setting the
camera mode to CaptureVideo.

Task-number: QTBUG-45659
Change-Id: I3692797128cfb70ba5ccbc7a36b6955471039e80
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>