Fix DismissView cannot be clicked sometimes after boot

mActivePointerId should be initialized to INVALID_POINTER.
Otherwise, the click event to DismissView can be intercepted by
onInterceptTouchEventScroll() due to wrong calculated value of yDiff
when mActivePointerId is not intialized.

This issue can be reproduced at the first click to DismissView while
user have not touched any notification views ever after boot.
And that click event must contains some MOVE events.

Test: manual - Check whether user always can click DismissView
               after boot.
Fixes: 31649735
Change-Id: If9baf6db1b891a58e17153806d52d395ddd661df

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

@@ -130,7 +130,7 @@ public class NotificationStackScrollLayout extends ViewGroup
     private boolean mIsBeingDragged;
     private int mLastMotionY;
     private int mDownX;
-    private int mActivePointerId;
+    private int mActivePointerId = INVALID_POINTER;
     private boolean mTouchIsClick;
     private float mInitialTouchX;
     private float mInitialTouchY;