The DJI library is one of the more complicated libraries available.
I hope that the next DJI SDK update will add support for AndroidX. Until then you will only be able to use it if you download the old ready to run SDK. I'm also not sure whether the current SDK supports Android 9 devices.
Bottom line is that it is probably better to wait for DJI to release a new SDK which will hopefully be supported by AndroidX and then wait for the updated B4A DJI library.