Rev B - 7/16/19
With the arrival of Android 8/Oreo, GnuRoot no longer runs on all Android devices, but there is a successor to GnuRoot, known as Userland. However, when I tried Userland, I gave up on even installing it because it requires knowledge that few users probably have, and the installer provides no indication of status or progress for the main installation, which is reportedly huge, which I gather means that it's larger than 100MB. But I don't know, because all of the articles written about it are vague too.
So, you could get an Android tablet which runs 7/Nougat, just for running Apt-offline on top of GnuRoot Debian, and noncritical apps. (Android security is apparently partly based on keeping the OS in a state of upheaval.) However, I can't guarantee that the resulting package-downloads would be safe to install, due to the lack of Android security updates, so use this approach at your own risk.
Another option for a portable device for running Apt-offline is a Chromebook, now that all Chromebooks run Linux apps:
In case you've missed it, last year, Google started making it possible to run desktop Linux on Chrome OS. Since then, more Chromebook devices are able to run Linux. Going forward, all of them will be able to do so, too. Yes. All of them. ARM and Intel-based.
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It's as simple as simple can be. Just open the Chrome OS app switcher by pressing the Search/Launcher key and then type "Terminal". This launches the Termina VM, which will start running a Debian 9.0 Stretch Linux container.
Congratulations! You're now running Debian Linux on your Chromebook.
Want Ubuntu instead? It's a bit more trouble, but you can bring up Ubuntu with a few shell commands. Want to run, say, Fedora? You can run Fedora with Chrome OS, as well.
from All Chromebooks will also be Linux laptops going forward
So, Chromebooks are now the ideal portable device for running Apt-offline, other than the fact that they don't fit in pockets, whereas small tablets fit in large pants-pockets. (I like Intel's compute-card design, where the electronics are put on a module and plugged into a lap-dock, so that the guts of the laptop could be removed and carried in a pocket. I'd also consider making the battery removable, if it's not already a standard feature, so that for example it could be put in a pocket instead of being left in a hot car.) Chromebooks are also the ideal approach to accessing the internet, whether or not you have good internet access at home, especially now that they can run Linux apps at the same time.
