Fixed a bug where the dimmed background could be invisible

Test: Notification group with 2 children on shade locked. Swipe 1 away and go to lockscreen
Change-Id: If7c3a55e220579eb27036f31d59b222371d2bf8e
Fixes: 31524202

packages/SystemUI/src/com/android/systemui/statusbar/ActivatableNotificationView.java

@@ -593,6 +593,9 @@ public abstract class ActivatableNotificationView extends ExpandableOutlineView
             public void onAnimationEnd(Animator animation) {
                 updateBackground();
                 mBackgroundAnimator = null;
+                if (mFadeInFromDarkAnimator == null) {
+                    mDimmedBackgroundFadeInAmount = -1;
+                }
             }
         });
         mBackgroundAnimator.addUpdateListener(mBackgroundVisibilityUpdater);