Fix crash on devices that don't support multiwindow

Only initializes split-screen organizer if device supports it.

Also added some graceful degredation for when organizer binding
fails. Needed to clean-up some things for this though since
now unregistering task-organizers needs to work.

Previously, registering one task-org for multiple windowing
modes worked, but unregistering only cleaned-up one windowing-mode.
So this reworks some of the data-structures to support that
use-case.

Bug: 152401027
Test: Use device that doesn't support split-screen multiwindow.
Change-Id: I7d417721b7b51b20b0c054d9a25f62c443837670