Connect systemwindow root with accessibility

This sets the most-recently created view on a shellroot
to be used as the "accessibility" window. This allows
each shellroot to have 1 window that accessibility sees.

Bug: 152368950
Test: atest AccessibilityWindowQueryTest#testWindowDockAndUndock_dividerWindowAppearsAndDisappears
Change-Id: Ice2820e11544ccdf7a3e600f918eac0ffb506548