Clicking on partially coverd views by other views or windows.

In touch exploration mode an accessibility service can move
accessibility focus in response to user gestures. In this case
when the user double-taps the system is sending down and up
events at the center of the acessibility focused view. This
works fine until the clicked view's center is covered by another
clickable view. In such a scenario the user thinks he is clicking
on one view but the click is handled by another. Terrible.

This change solves the problem of clicking on the wrong view
and also solves the problem of clicking on the wrong window.
The key idea is that when the system detects a double tap or
a double tap and hold it asks the accessibility focused node
(if such) to compute a point at which a click can be performed.
In respinse to that the node is asking the source view to
compute this.

If a view is partially covered by siblings or siblings of
predecessors that are clickable, the click point will be
properly computed to ensure the click occurs on the desired
view. The click point is also bounded in the interactive
part of the host window.

The current approach has rare edge cases that may produce false
positives or false negatives. For example, a portion of the
view may be covered by an interactive descendant of a
predecessor, which we do not compute (we check only siblings of
predecessors). Also a view may be handling raw touch events
instead of registering click listeners, which we cannot compute.
Despite these limitations this approach will work most of the
time and it is a huge improvement over just blindly sending
the down and up events in the center of the view.

Note that the additional computational complexity is incurred
only when the user wants to click on the accessibility focused
view which is very a rare event and this is a good tradeoff.

bug:15696993

Change-Id: I85927a77d6c24f7550b0d5f9f762722a8230830f