Fixed the transition of the background of the notifications

When there's no notification anymore, the background could animate
weirdly as it still factored in the stack translation, which doesn't
make much sense. This lead to weird transitions on the lockscreen.

Change-Id: I0f17dc5f667106dcad54702be1880a689df24764
Fixes: 30341287

packages/SystemUI/src/com/android/systemui/statusbar/stack/NotificationStackScrollLayout.java

@@ -2009,7 +2009,7 @@ public class NotificationStackScrollLayout extends ViewGroup
                 bottom = Math.min(bottom, getHeight());
             }
         } else {
-            top = (int) (mTopPadding + mStackTranslation);
+            top = mTopPadding;
             bottom = top;
         }
         if (mPhoneStatusBar.getBarState() != StatusBarState.KEYGUARD) {