On Android, the closest thing to an "app building app" is Tasker. It's meant for end-user automation, scripting without much typing, but it's powerful enough to build whole GUI apps (and it wisely offers an option to export such creations as standalone .apks).
It's definitely not Hypercard, but it shares its spirit, and is still a very useful thing. Like, e.g. last weekend I though I'd like to run a sampling profiler on myself - i.e. every now and then pop up a notification to record what I'm doing at any given moment, and store that data in a format I could later use for analysis. I managed to build one in 30 minutes in Tasker - complete with showing the list on a smartwatch and recording to CSV; after first spending the same amount of time looking for existing solutions and failing to find any. That's to show how easy it is to do, straight on your smartphone. It's something you could easily do on a bus or a train - and more importantly, it's something a regular non-tech person could do too.