Any way to get Android TV on my TV to activate the screensaver when on a HDMI source?
Currently any HDMI source overrides the TV's screensaver. Is this just not possible because Android TV can't tell if a device connected to HDMI is being actively used?
Correct. It detects activity by the Android OS, it isn't scanning the HDMI signal for changes. Your other device needs its own screensaver. You might be able to write up some kind of Tasker script to automatically change the input back to Android after a certain amount of idle time.