the notifications are popping up almost instantly ( despite me providing time for it ).
you pass the time you want it to be triggered from now on. The API expects a time since EPOCH (as your comment in the code also states) which are the milliseconds from 1970/01/01 on.
So in your JAVA code add new Date().getTime() milliseconds to your current timeout.
So far i am able to get the notifications ( but not on Android O )
did you read this already?
Alos chekc the Qt debugger console and/or LogCat for suspicious messages regarding notifications.