KDE and Gnome are desktop environments that runs on top of an OS, they themselves are categorically not OS, and they don't provide OS facilitates. They don't even directly talk to RAM or core hardware without an OS managing everything in the middle.
Android is all by itself an OS, it got everything an OS should have, scheduler/trap handling, runtime, system apis/syscalls all the good stuff. It's just happened that Android currently runs on top of Linux kernel, and it utilizes the kernel to provide some aforementioned facilitates, so yes you can say Android builds On Top of Linux, but that's not saying Android Is Linux, the sentences simply mean different things.
