http://stackoverflow.com/questions/10953708/is-it-possible-in-android-to-have-the-camera-focus-on-a-specific-point-in-the-pr
In Android 4.x, this is possible, with the setFocusAreas. You'll have to check getMaxNumFocusAreas first to see if this feature is supported on your device, and how many areas to use.
Then, you'll need to convert the user's touch coordinates to the coordinates used by the Camera.Area object (described here), and call setFocusAreas with the coordinates. From then on, calls to autoFocus will use that region for selecting focus.