Resets All Apps pullback alpha on state change.

Previously the progress alpha was updated, but in some cases (e.g.
0 animation speed), the final alpha was not always applied to the
pullback property.

Fix: 243015279
Test: Disable animations, swipe up to all apps, swipe up nav bar
to close, swipe up to all apps again. Alpha is now properly set.
(See b/232536048#comment49).

Change-Id: Ifb8a4cd3ee7cc94a5851d355fdf52860366b8f2f