sysui: refactor for extensibility

Make mQsExpansionEnabled protected in NotficationPanelView.

Bug: 27836776
Change-Id: I35479e990607d6801effd3fa64fd3ffb1f9503dd
(cherry picked from commit 8af5736425d60641268943fe4d7e23a2b7d99c8a)
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 d9a0f43..7486519 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationPanelView.java
@@ -136,7 +136,7 @@
     private boolean mStackScrollerOverscrolling;
     private boolean mQsExpansionFromOverscroll;
     private float mLastOverscroll;
-    private boolean mQsExpansionEnabled = true;
+    protected boolean mQsExpansionEnabled = true;
     private ValueAnimator mQsExpansionAnimator;
     private FlingAnimationUtils mFlingAnimationUtils;
     private int mStatusBarMinHeight;