Add new hover move action and scroll wheel plumbing.

Added support for tracking the mouse position even when the mouse button
is not pressed.  To avoid confusing existing applications, mouse movements
are reported using the new ACTION_HOVER_MOVE action when the mouse button
is not pressed.

Added some more plumbing for the scroll wheel axes.  The values are
reported to Views but they are not yet handled by the framework.

Change-Id: I1706be850d25cf34e5adf880bbed5cc3265cf4b1