Accessibility to toggle multiwindow mode

Encapsulating the logic to toggle multiwindow mode from recents,
and plumbing it through to accessibility global actions. Sending
accessibility events when windows bounds change. Exposing the
dock divider window type to accessibility services.

Bug: 27250995
Change-Id: Ib7491f1f853dc7f01bf5c5a4ac1f914f55d0608a