Update bottom area alpha when stack height changes

Bottom area alpha depends on stack scroller's notification top Y
which depends on stack translation which changes when stack height
changes. Thus the bottom area alpha needs to be updated when stack
height changes.

Bug: 20457060
Change-Id: Ibcc95c830a83737732aa15147cd82d21f2ee9161
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
index b87c25b..f8cb689 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
@@ -327,7 +327,7 @@
         } else if (!mQsExpanded) {
             setQsExpansion(mQsMinExpansionHeight + mLastOverscroll);
         }
-        mNotificationStackScroller.setStackHeight(getExpandedHeight());
+        updateStackHeight(getExpandedHeight());
         updateHeader();
         mNotificationStackScroller.updateIsSmallScreen(
                 mHeader.getCollapsedHeight() + mQsPeekHeight);
@@ -1457,7 +1457,7 @@
             setQsExpansion(mQsMinExpansionHeight
                     + t * (getTempQsMaxExpansion() - mQsMinExpansionHeight));
         }
-        mNotificationStackScroller.setStackHeight(expandedHeight);
+        updateStackHeight(expandedHeight);
         updateHeader();
         updateUnlockIcon();
         updateNotificationTranslucency();
@@ -2241,4 +2241,9 @@
         mScrollView.setTranslationX(translation);
         mHeader.setTranslationX(translation);
     }
+
+    private void updateStackHeight(float stackHeight) {
+        mNotificationStackScroller.setStackHeight(stackHeight);
+        updateKeyguardBottomAreaAlpha();
+    }
 }