Add MotionEvent.HOVER_ENTER and HOVER_EXIT.

The input dispatcher sends a HOVER_ENTER to a window before dispatching
it any HOVER_MOVE events.  For compatibility reasons, the window will
*also* receive the HOVER_MOVE.  When the pointer moves into a different
window or the pointer goes down or when events are canceled for some reason,
the input dispatcher sends a HOVER_EXIT to the previously hovered window.

The view hierarchy behavior is similar.  All views under the pointer
receive onHoverEvent with HOVER_ENTER followed by any number of HOVER_MOVE
events.  When the pointer leaves a view, the view receives HOVER_EXIT.
Similarly, if a parent view decides to capture hover by returning true
from onHoverEvent, the hovered descendants will receive HOVER_EXIT.

The default behavior of onHoverEvent is to update the view's hovered
state by calling setHovered(true/false).  Views can query their current
hovered state using isHovered().

For testing purposes, the hovered state is mapped to the pressed
drawable state.  This will change in a subsequent commit with the
introduction of a new hovered drawable state.

Change-Id: Ib76a7a90236c8f2c7336e55773acade6346cacbe