AVFoundation: correctly detect the default audio capture device.

Use AVCaptureDevice::defaultDeviceWithMediaType instead of the first
device in the list of available devices.

Change-Id: I436921f99280a28d7158d345cd977a874cfb8968
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>

src/plugins/avfoundation/camera/avfaudioinputselectorcontrol.h

@@ -69,6 +69,7 @@ private:
 
     QString m_activeInput;
     bool m_dirty;
+    QString m_defaultDevice;
     QStringList m_devices;
     QMap<QString, QString> m_deviceDescriptions;
 };