Sad news on Android 10 :-(
I have now investigated some workarounds.
First way SYSTEM_ALERT_WINDOW:
This is an really old permission and will be removed from Android in the next release.
!!IMPORTANT UPDATE!!! [2019-05-08] We all SAW it coming (pun intended), but the Android team have finally given us an idea of where the out-of-app UI interaction that system-alert-window enabled is going. Confirmed at Google IO 2019: it's going away. Part of the flakily-recorded "Whats New In Android" talk mentions it here.
Google has decided to eventually deprecate SYSTEM_ALERT_WINDOW in a future Android release.
And Android Go devices will no longer grant this permission as of now.
So I can't use this event it would work (not tested it because of the deprecation) :-(
Second way FULL SCREEN INTENT:
Seems promising but again not intended to use it as an "dialog popup".
I have changed my code and have tried to show it as "FullScreenIntent".
But it will not always show the intent/popup.
The system decides if it will show it or not (as mentioned also in the official developer docs linked above).
In my tests I have seen a heads up notification at the top, but not the COL Reminder Popup Dialog.
You can compare it like the same if you get a call if you are inside an another app.
You see a notification hanging from the top where you can "CALL" or "DISMISS".
And it looks the same for the notification from COL Reminder.
Only if the screen is off I see the COL Reminder Popup after I switch the screen on..
So this solution is also not exactly the same but I think the best.
Solution/Alternative for Android 10:
See first post for the currently available soltuion.