A follow up to my own question. I used the horrible Android Studio to generate me an empty activity project. Then I added processing-core.jar from android mode. I add code to start a sketch (https://android.processing.org/tutorials/android_studio/index.html) which called a class in my Scala jar file. It worked just fine.
I compared the file build.gradle which was generated from processing-android IDE and another one generated from the horrible Android Studio. The only different is processing-android uses ejc to compile instead of javac from JDK 8.
So I wonder if there is a way to configure processing-android IDE to use javac from JDK8?
psksvp