Once the app is all set up on the Android device, use double-stick tape to affix it to one corner of the mirror.

Use an opaque paper to block out the rest of the back of the mirror with more double stick tape. I then used packing tape to affix some cardboard spacers that would bring the rest of the mirror to the same thickness as the tablet, so it will sit flat on the wall. 

Then use some velcro tape or your preferred mounting mechanism to affix it to the wall.