The situation your in essentially is that you have more manpower available during the first part of the sprint. Its the same if some developers would go on holiday: The work power is reduced after that point. This will result in a slower velocity for the second part of your sprint. You can reflect this in the burndown chart!
Let's assume your workers work with 10 units/week. Then you will get 20 units done in the first week and 10 units done in the 4 subsequent weeks. When you draw the assumed velocity into the burndown, you can consider these numbers. Then you will still see whether or not you're on schedule.
Anyways, I think Sklivvz has a point here... what's you Android Developer going to do during the rest of the time? Twist thumbs?