Is there a way to use an Android Mobile as a focusing screen for Nikon D800?

You may plug it into the Android device via USB (right cable or adapter USB-A to your mobile USB port). Then install a camera control app, like Camera connect & control or Helicon. The D800 is listed as supported by those, but check well if live view actually works for your model. The Nikon's control app, Snapbridge, officially doesn't support the D800.

You may also try with the HDMI camera output: get a HDMI to USB adapter, plug it on the android and get some app "Android as USB monitor". You should see the live view, but cannot control the camera.