There are two main parameters in screen geometry: Size and density. You can deal with the major size differences across tablets and handsets with Fragment objects and two or three different layouts per Activity, keyed to screen size, and two or three sets of assets, keyed to density. So at most you have 9 combinations, usually 6 or fewer, to test. This will cover 90%+ of non-game cases. Portability for OpenGL games is a whole other universe.
Also: Use RelativeLayout. Get your designer to design directly in the layout editor, and test them on screens he finds important and representative. That way he won't specify designs that can't be implemented.
Also: Use RelativeLayout. Get your designer to design directly in the layout editor, and test them on screens he finds important and representative. That way he won't specify designs that can't be implemented.