That's normal. If no other app is using battery, obviously Android would be the biggest. Remember it's a 'relative' battery consumption of an app since the last time you were at full battery. Android is not using 20% of your battery, it used 20% of your consumed battery. You have 34% left, so you used up 66%, and 20% of that is about 13%. So in 16hrs, Android OS used 13% of battery, which is actually quite good I'd think.