I see no reference to that file in a checkout of the upstream Linux kernel I had here, and I never saw that file in /proc in any Linux system I've played with,

I see no reference to that file in a checkout of the upstream Linux kernel I had here, and I never saw that file in /proc in any Linux system I've played with, but looking at my phone (with Termux) I do see that file. I agree, it seems to be Android specific.