I would suggest posting this question in the After Effects forum.
To answer the question… Use a clear piece of glass. Mount the glass firmly in place between the camera and subject. Lock everything off. If there is any movement of the tablet relative to the camera, then multiply your budget times 10, and add five weeks to the post timeline.
You need to think about what will be in focus. The surface of the tablet or the talent? If the tablet, you will need something to focus on. Temporarily place something on the glass surface (like the talent’s finger), get the focus, then remove it. DO NOT put any tape on the glass as you will spend hours/days trying to remove it in post. Have the talent wash their hands and keep the A/C on so they don’t sweat. Have a professional do a manicure on the talent. Finger nails look horrible in close ups.
If the person is moving the tablet, DO NOT put tape on the glass (have I said that yet?). Use the persons hand as a trackable object. Have them hold the glass firmly with one hand.
If you are using sharp-edged cut glass (like the glass you find in that picture frame over there), have a medic on hand with lots of bandages, and make sure your insurance is up to date.
Oh… and don’t put any tape or track markers on the glass.