That's the moment your OS becomes a "mobile app platform", or a "software distribution". Or just the definition of OS changes. Or maybe not, because Windows was always more than the kernel. And it always included Notepad. But Notepad was never essential. And traditional UNIXes always came with a lot of user space programs. (And thre are essential user space apps for POSIX-y OSes, right? Things like the coreutils programs, ls, stat, cut, test, rm, [, sh, etc.)

And there's the WebView component, that's part of the SDK, that's essential, and it can be a dependency, or it can be bundled into the APK. (There's even https://crosswalk-project.org/ that is basically new WebView for old phones - Chromium built for old phones.)