This is a self ask and answer article, hoping to benefit somebody else.
I have an iPhone app and want to enable it for both iPads and iPhones (as a Universal app). I've updated the app general app drop down menu to set my app as Universal app, but the Asset catalog only specifies iPhone icons.
I search for many suggestions here, including inserting the list via the Info.plist and event the alternative app icon API. Not really what I had in mind, and of course, they didn't work the way I had hoped.
How to get the iPad app icons to show in the Assets.xcassets?
So, to solve this problem, I copied the .png icons of the Assets.xcassets/AppIcon.appiconset folder to a new folder on my desktop.
Then I selected the Assets.xcassets folder in the Xcode (9.1) navigation viewer and then found the AppIcon object and deleted it.
Going back to the project in Xcode navigation and visiting the general view of my target, I scrolled to the App Icons Source drop down, and selected the AppIcon. I had to fiddle (select, deselect, select...) with this a little since Xcode was slightly confused, but eventually, it GOT IT!
Then, I went to the assets.xcassets folder and found the blank AppIcon object and dragged in my various icons onto their specific-sized placeholders.
Say bye to the app-in-progress icon.