Many of the samples out there tell to use Highgui module to read image with OpenCV. There is no Highgui module in OpenCV 3.0+, at least not in Java/Android. I found out this from http://stackoverflow.com/a/25943085/5749443
Migration from OpenCV 2.X to 3.X (Java)
Highgui.imread(fileName, Highgui.CV_LOAD_IMAGE_GRAYSCALE) Highgui.imread(fileName)
Imgcodecs.imread(fileName, Imgcodecs.CV_LOAD_IMAGE_GRAYSCALE) Imgcodecs.imread(fileName)
Most of the Core-stuff is at Imgproc. Remember to check videoio module as well.