Can I use Apple emoji in App Store icons?
I would say no. 

Apple review guidelines state that you must own the rights for any images you display in, or on, your app. https://developer.apple.com/app-store/review/guidelines/

Guideline 2.3.9 states:


  2.3.9 You are responsible for securing the rights to use all materials in your app icons, screenshots, and previews, and you should display fictional account information instead of data from a real person.


Also, Apple gives violations of this guideline as one of the top reasons for rejection and/or removal from the App Store. If you include artwork that you don't own, you might get thru Review but they're getting stricter and might remove it.