Fix checking crop region for home/recents

Wasn't handling split-screen properly. Now it checks if the
home/recents is the top of its "visibility" stack vs top
of root-task. This means it must be the top sibling in either
the displayarea or the root-split. This check is necessary
for dealing with visible windows that are behind a translucent
task.

Bug: 159138687
Test: Enter split with 2 apps, close the top app, return to home
      and verify that it receives touches in top half
Change-Id: Ie6fed2dd945952b83116721a98fc6d7c8396729f

• services/core/java/com/android/server/wm/Task.java[diff]
• services/core/java/com/android/server/wm/WindowState.java[diff]