Enhance computation of click point for accessibility.

In explore by touch mode the user performs a double tap to click on
an item. In this case the system sends down and up events at the
location of accessibility focus. The accessibility focused view may
be partially covered. In order to click in this view we compute a
point where to send the down and up events. This clicking strategy
is a bridge-gap and we will switch to accessibility actions in the
future.

When computing the point to click we were taking into account whether
the view was covered by a clickable sibling or a clickable sibling of
a predecessor. Despite our expectation cases in which this is not
enough happen in practice. In particular, the focused view may be
covered by a clickable descendant of a non-clickable sibling of a
predecessor that covers the focused view. This change takes care
of handling this case. Note that computing the click point is a fair
amount of work but this happens very rarely and on demand. Also the
code is short circuiting where possible.

Change-Id: I4d3cd8b67a7baf0bcc12f370ea7ba1b04c42c355