Detecting a hand on touch screen

I'm trying to make a "fake handprint scanner" on an Android tablet, similar to this for iOS: https://itunes.apple.com/gb/app/palm-scan/id364619541

My problem is that while the various Input.touch methods work when or more unique finger touches are detected, when placing the whole palm of a hand on the screen, Input.touchCount is 0. (as tested on a Galaxy Tab, Samsung S7 and Xperia)
Is there any way (perhaps using a native Android plugin?) to detect continuous skin contact anywhere on the screen? I'm not bothered about trying to read a position, since the hand outline I'm expecting players to match fills up basically the whole screen anwyay.

Thanks in advance for any suggestions.

 

Hey, sorry, i'm years too late in here .
Yeah, i'm also wondering how to do something like that on Unity, just to detect the hand and that's it.
I'm leaving a photo as a reference.