base

Add support for arbitrary axes in MotionEvents.

This change makes it possible to extend the set of axes that
are reported in MotionEvents by defining new axis constants.

The MotionEvent object is now backed by its C++ counterpart
to avoid having to maintain multiple representations of the
same data.

Change-Id: Ibe93c90d4b390d43c176cce48d558d20869ee608