Improve the locked-shade close animation after dismissing a single notification

Wait for the child animations to finish before we close the shade,
otherwise the notification outro animation will start in the shade, and
finish on the lockscreen which causes a flicker.

Fixes: 388298358
Test: Dismiss the last notification in the locked shade -> observe the
animations
Flag: com.android.systemui.notification_shade_close_waits_for_child_animations

Change-Id: Ie670bc5906603276b1425133d84c1fdd168eb6ef