workaround to wrong fling direction for android P: facebook#21117

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java

@@ -299,6 +299,16 @@ public void getClippingRect(Rect outClippingRect) {
 
   @Override
   public void fling(int velocityY) {
+    // Workaround.
+    // On Android P if a ScrollView is inverted, we will get a wrong sign for
+    // velocityY (see https://issuetracker.google.com/issues/112385925). 
+    // At the same time, mOnScrollDispatchHelper tracks the correct velocity direction. 
+    //
+    // Hence, we can use the absolute value from whatever the OS gives
+    // us and use the sign of what mOnScrollDispatchHelper has tracked.
+    velocityY = (int)(Math.abs(velocityY) * Math.signum(mOnScrollDispatchHelper.getYFlingVelocity()));
+
+
     if (mPagingEnabled) {
       flingAndSnap(velocityY);
     } else if (mScroller != null) {