Fix issues with SHOW_WHEN_LOCKED windows

- Restore the previous logic to only show SHOW_WHEN_LOCKED windows
if appShowWhenLocked is not null.
- Add new logic to keep previous SHOW_WHEN_LOCKED window around if
it's still animating, as well as all the other windows that are
still visible.

Test:
- Make sure an SYSTEM_ALERT_WINDOW with SHOW_WHEN_LOCKED isn't
visible on the lockscreen.
- In Google Camera, click on the lock icon after launching it above
lockscreen.

Change-Id: Id33a8008c6edd6272e5110d9c0f1bf7aab007564
Fixes: 31300204
Fixes: 31319061