Don't do what specifically ? and why not ? and what should I do instead ?
All of these answers are missing in your comment.
WhatsApp is doing something quite similar - displaying a small search icon. When you click on it - the search box expands to 100% (with animation, of course) and the keyboard input pops up.
But for example - on map based apps I would personally prefer to have the map with as little distractions (icons, buttons,text inputs, labels) as possible.
In Google's Android Chrome the url box is hidden once you start scrolling down. I like that . And they probably have the data to show you that other people like that as well, because this 'hide-when-scrolling-down' feature is there at least a year or so, If I remember correctly.