Fix jank in launcher

The icon grid was reappearing at the end of the animation, causing
slower frame draws at the end of the animation, possibly leading
to jank.

Fix this by waiting with resetting the alpha until the whole
animation is done.

Test: Open/close apps
Bug: 75985430
Change-Id: I8fa62c5f648335ce9d4c4450d52c46465e2d08bf