First stab at poly-finger support.

The MotionEvent API should be fairly solid, but there is still a lot of
work to do in the input device code.  In particular, right now we are
really stupid about watching how fingers change -- we just take whatever
the driver reports as down and dump that directly into the motion event.

The big remaning work is to assign pointer IDs so that applications have
help in determine which fingers go up and down, and adding support for
the official multi-touch driver protocol.