Android: How to maintain aspect-ratio in animation

The animation I am running inside an imageview refuses to maintain the aspect-ratio of the image frames. The following answers in SO are quite informative, but don't seem to work for me: How to scale an Image in ImageView to keep the aspect ratio

Here is the code:

private void startAnimation(){
    mImageView.setAdjustViewBounds(true);
    mImageView.setScaleType(ScaleType.CENTER);
    mImageView.setBackgroundResource(R.anim.my_animation);

    AnimationDrawable frameAnimation = (AnimationDrawable) mImageView.getBackground();

     // Start the animation (looped playback by default).
     frameAnimation.start();
}

R.anim.my_animation is just an animation list:

    ... and so on...