Android 8 - doesn’t support AR applcations from Unreal Engine, all phones no longer support existing AR applications due to broken camera bug (black-screen). Please do not update your Android 7 to Android 8, I currently facing this issues and many others, we are discussing about it on this thread:
https://forums.unrealengine.com/development-discussion/vr-ar-development/1450750-ar-samsung-s8-after-upgrade-to-android-8-stopped-work-with-ue4-ar-apps