I know there is a lot of similiar questions to this, but a lot of them seem to be out of date because of the development of libraries such JavaCV.
I used the code from
Update! I've also found out that if you have have an armeabi-v7a folder in the jniLibs, or are using anything other than an armeabi folder, you will have issues with the .so files not being added.
Looks like ffmpeg is not being imported. I had the same problem. Here is what I did. Let me know if this helps you!
compile group: 'org.bytedeco', name: 'javacv', version: '0.9'
compile group: 'org.bytedeco.javacpp-presets', name: 'opencv', version: '2.4.9-0.9', classifier: 'android-arm'
compile group: 'org.bytedeco.javacpp-presets', name: 'ffmpeg', version: '2.3-0.9', classifier: 'android-arm'