Haven't tried the APK yet, but I've noticed that when I ran the AR demo just now for the first time since a while, on my Samsung Galaxy S7 edge, it appeared as if didn't work:
Gave black background screen with controls over it and some text, nothing else.
However when I ran it the 2nd time after couple mins then it all went fine and kept working fine even after closing and restarting AGK.
Perhaps android still needs to install some AR-Core related stuff, as I believe they updated it a while ago?
[EDIT]
Just checked on my phone under settings>apps
select from top left menu: show system apps
Then it shows ARCore> version 1.6.181112096
Android version is: 8.0.0