Corrected issues with multi-touch support in buttons.

This is my attempt at correcting multi-touch behavior of buttons on Android. The original implementation did not take the touch ID into consideration and as a result was firing events for all buttons instead of the one associated with a particular touch. 

One particular thing to note is that this changes the behavior of buttons slightly - the only way a button is pressed is if the touch down happens on the button itself. Before these changes, you could touch somewhere, then drag your finger over to a button and it would register the button as pressed. This is no longer the case.