I found the culprit. I changed the minSDKVersion in my_opencv_sdk_path\sdk\build.gradle from its original value of 21 to 19 because I wanted to match against my project minSDKVersion of 19 (I wanted to support more legacy devices).
But now because of this problem, I set my project minSDKVersion to 21 and return the OpenCV minSDKVersion back to 21. Everything goes well without problem.