Avoid saving position before minimizing when in transition

Changed to save snap targets instead of positions when divider is
attached to window and after any stopdragged animations occur.

Test: manual - split 2 apps, rotate screen, hold home launch assistant,
click keyboard, hold device back to portrait and press home
Fixes: 64977831
Change-Id: If47a04742b812eed3be0d08a243d4226515308b5