EDIT:
So according to this thread, this is a Google related change and not Nvidia?
In which case, similar to the banner issue from a previous update, you can get a different launcher (see comments for FLauncher) to avoid it
https://www.reddit.com/r/ShieldAndroidTV/comments/181vh4t/ads_at_full_screen/
YES! FINALLY! i thought something was missing from this expensive piece of hardware and here it is, ads! I love wasting time on ads and they sure know what i love!
/s