Not touchable windows should not hide windows behind

When computing which windows to report for accessibility we were not
reporting windows behind non touchable ones. However, if a window is
not-touchable the touch events go to the window behind it. Hence, a
not-touchable window should not affect the reproted windows for
accessibility.

Change-Id: I213a32b33b0986a73d8bbdf0a3e40e6ca6e4261b