@Override
                public Boolean call() throws Exception {
                    try {
                        // try...try...try
                        Camera camera = Camera.open();
                        camera.release();
                    } catch (Exception e) {
                        BoxingLog.d("camera is not available.");
                        return false;
                    }
                    return true;
                }
start system camera to take a picture

@param activity      not null if fragment is null.
@param fragment      not null if activity is null.
@param subFolderPath a folder in external DCIM,must start with "/".
boxing/src/main/java/com/bilibili/boxing/utils/CameraPickerHelper.java