Fix end-aligned short-shelf animation

Fix the issue that the new icon was appearing from the left
side of Stack instead of the left boundary of the notification
shelf when the shelf if aligned to the end of screen.

Test: manual, dismiss the last notif while shelf showing
Fix: 411261819
Flag: com.android.server.notification.notification_minimalism
Change-Id: I52fa58e73e607a3b2e1f52890941348114a2b601