BB10: Qt Quick Camera fails to initialize sometimes

After applying patch from QTBUG-43752 it becames possible to use Qt Quick Camera and show preview.

However camera initialization may fail randomly when FrontFace camera is used by default (available since Qt 5.4):

    Camera {
        position: Camera.FrontFace
    }

It seems that QCamera uses back camera as default device and when front camera is specified as default device we will get the following behaviour:

• QCamera::start is called for back camera
• QCamera::unload is called for back camera
• QCamera::start is called for front camera

The latest call fails randomly in BbCameraSession::startViewFinder on this check:

    if (result != CAMERA_EOK) {
        qWarning() << "Unable to start viewfinder:" << result;
        return false;
    }

My workaround is to call applyConfiguration directly in BbCameraSession::setState instead of queueing it with invokeMethod.
I have not found any regression after this change and I see no reason why we must use invokeMethod