Use Divider ShellRoot to ferry divider to Accessibility

This is kinda a hack until we have more robust WM and
accessibility infrastructure in place. It assumes that the
shell-root for docked divider only has one window in it.

Bug: 152368950
Test: Enter split-screen with Talkback on. Touch divider.
Change-Id: I401c6f889540054a8ee691bea714dc7a1b1658c8