On All android devices except on Android 9 after the AD is loaded I was able to change the width of the PublisherAdView to go fullscreen width by doing the following
publisherAdView is the object of the PublisherAdView
int width = Util.getDisplayWidthInDp(publisherAdView.getContext());
publisherAdView.setAdSizes(new AdSize(width,80));
above code will resize the publisherAdView to fit full width but on Android pie devices publisherAdView is not resizing.
I also tried
width =-1
It might work but DFP is throwing an error saying "-1x80" is too small to flight.
Is there a way to overcome this issue?