Fix AccessibilityNode's isVisibleToUser behavior.

The isVisibleToUser property of an AccessibilityNodeInfo specifies
whether the user can see the source view. It is used by accessibility
services to figure out whether to focus on a view. This property
was giving a wrong value if the view is covered by another window
such as the keyboard. As a result the user hears one thing but when
double taps interacts with the overlaid window which is another thing.

bug:15938254

Change-Id: Ib9feb20ea422a24a512c47ed1234961ae0386a7f