Fix Inverted Horizontal ScrollView on Android (facebook#23233)

Summary:
ScrollView (as well as FlatList) used with horizontal={true} and inverted={true} do not scroll as expected due to a known Android P bug.

Fixes facebook#22710. This is pretty much a clone of PR facebook#21117, applied to a horizontal ScrollView.

[Android] [Fixed] - Fix Inverted Horizontal ScrollView on Android

1. Create a test application with a FlatList with horizontal={true} inverted={true}.
2. Fill it with data
3. Scroll and release your finger
Pull Request resolved: facebook#23233

Differential Revision: D13915911

Pulled By: cpojer

fbshipit-source-id: d32c82e6d9076f5ffdf342fcd71bd921e9c8a97b

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

@@ -286,8 +286,18 @@ public boolean onTouchEvent(MotionEvent ev) {
 
   @Override
   public void fling(int velocityX) {
+
+    // Workaround.
+    // On Android P if a ScrollView is inverted, we will get a wrong sign for
+    // velocityX (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.
+    final int correctedVelocityX = (int)(Math.abs(velocityX) * Math.signum(mOnScrollDispatchHelper.getXFlingVelocity()));
+
     if (mPagingEnabled) {
-      flingAndSnap(velocityX);
+      flingAndSnap(correctedVelocityX);
     } else if (mScroller != null) {
       // FB SCROLLVIEW CHANGE
 
@@ -302,7 +312,7 @@ public void fling(int velocityX) {
       mScroller.fling(
         getScrollX(), // startX
         getScrollY(), // startY
-        velocityX, // velocityX
+        correctedVelocityX, // velocityX
         0, // velocityY
         0, // minX
         Integer.MAX_VALUE, // maxX
@@ -316,9 +326,9 @@ public void fling(int velocityX) {
 
       // END FB SCROLLVIEW CHANGE
     } else {
-      super.fling(velocityX);
+      super.fling(correctedVelocityX);
     }
-    handlePostTouchScrolling(velocityX, 0);
+    handlePostTouchScrolling(correctedVelocityX, 0);
   }
 
   @Override