I have been looking into this myself, with no conclusive information on how to reliably accomplish this. It feels like Qt 5.6 ought to automagically do the scaling for you; I intend to post a new question in this forum asking about this. In the meanwhile, the first part of this video will be of interest to you:
There is code at 06:24 that extracts the dpi value from the Android framework. Maybe that will work better, I have not tried it myself.