Interface change: the test now uses activity touch events (ACTION_DOWN) rather than onClick events on the button. This should make reaction times faster because it doesn't wait for the touch to stop before calculating reaction time. Also, it is possible to touch anywhere on the screen other than the title or status bar rather than just on a specific "button".
Refactoring: rename ReactionTestObserver callback interface to ReactionTest.Callback. Minor renaming of methods, adding comments