Only handle onHoverEvent in actionable views.

Modified onHoverEvent so that a view only has its state changed to
hovered when the view is clickable.  The intent is to prevent labels
from taking hover focus away from their containing view group despite
not being actionable in any way.  The child will still receive
a hover event but, if it does not handle the event (returns false),
the hover event will bubble up to its parent and ancestors allowing
them a chance to handle the event instead.

The new onHoverEvent semantics are better because now they closely
mirror those of onTouchEvent.  This makes it straightforward to
implement views that respond to hover by changing their visual
appearance (such as by making buttons glow when hovered).

Added onInterceptHoverEvent to enable ViewGroups to explicitly
intercept hover events within their bounds.

Exposed the new hover event API for real.

Change-Id: I63195c8f5c74b859f6047487f9a0f703e8f40ffe