sysui: refactor for extensibility.

Mafke mDismissView in NotificationStackScrollLayout
protected.

Bug: 28848771
Change-Id: I3d0bb136835c3e21f55d103672744cc375e2d1c9
(cherry picked from commit 97192ff2c38d2362ebd449ec329c2da66644bec2)
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/stack/NotificationStackScrollLayout.java b/packages/SystemUI/src/com/android/systemui/statusbar/stack/NotificationStackScrollLayout.java
index ccc97e6..a6f3baf 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/stack/NotificationStackScrollLayout.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/stack/NotificationStackScrollLayout.java
@@ -194,7 +194,7 @@
     private boolean mExpandingNotification;
     private boolean mExpandedInThisMotion;
     private boolean mScrollingEnabled;
-    private DismissView mDismissView;
+    protected DismissView mDismissView;
     protected EmptyShadeView mEmptyShadeView;
     private boolean mDismissAllInProgress;