I happen to have a mac that was syncing Screen Time data across, and I took a dive on there to see if anything matched.
com.apple.secureelementservice.aapp.plist exists on the system, as a user preference property list, with the only property appearing to be SBIconVisibility (set to false for me), which I can only imagine refers to an icon in the status bar. This would most likely be tied to some apple pay thing, though it showing up as an app in the App Library on iOS is absolutely not something I can find anything on, and weird that it happened.
After disabling lockdown, letting the app activity sync back to my iOS devices, i see the AApp come back in the Misc section, which I speculate to be something running on my mac. given the nature of the plist (despite it being in userland) i suspect AApp stands for "Apple App" internally, though I haven't seen used anywhere else like that.
I hope this helps someone! I would love a more concrete answer from someone at Apple familiar with the matter!