added in snapAlignment for horizontal android scrollView

Summary:
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`snapToAlignment` is available on iOS but not android yet. This PR is to add support for `snapToAlignment` on android as `snapToInterval` was recently added to android and they are very useful together.

Make a `Flatlist` in android with `pagingEnabled`, `horizontal`, `snapToInterval` and `snapToAlignment` set and see how adjusting between the three values of `snapToAlignment` aligns just like it does in iOS.

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[ANDROID] [MINOR] [ScrollView] - On Android, **ScrollView** now takes snapToAlignment like iOS

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  **INTERNAL and MINOR tagged notes will not be included in the next version's final release notes.**

    CATEGORY
  [----------]      TYPE
  [ CLI      ] [-------------]    LOCATION
  [ DOCS     ] [ BREAKING    ] [-------------]
  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
  [ INTERNAL ] [ ENHANCEMENT ] [ {Filename}  ]
  [ IOS      ] [ FEATURE     ] [ {Directory} ]   |-----------|
  [ ANDROID  ] [ MINOR       ] [ {Framework} ] - | {Message} |
  [----------] [-------------] [-------------]   |-----------|

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
Closes facebook#18648

Differential Revision: D7473762

Pulled By: mdvacca

fbshipit-source-id: ad4778b83f9fd1352455b2ed28a5f37229d9d8c7

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

@@ -52,6 +52,7 @@ public class ReactHorizontalScrollView extends HorizontalScrollView implements
   private @Nullable Drawable mEndBackground;
   private int mEndFillColor = Color.TRANSPARENT;
   private int mSnapInterval = 0;
+  private String mSnapAlignment = "start";
   private ReactViewBackgroundManager mReactBackgroundManager;
 
   public ReactHorizontalScrollView(Context context) {
@@ -96,6 +97,13 @@ public void setPagingEnabled(boolean pagingEnabled) {
 
   public void setSnapInterval(int snapInterval) {
     mSnapInterval = snapInterval;
+    if(snapInterval != 0) {
+      mPagingEnabled = true;
+    }
+  }
+
+  public void setSnapAlignment(String snapAlignment) {
+    mSnapAlignment = snapAlignment;
   }
 
   public void flashScrollIndicators() {
@@ -236,6 +244,17 @@ private int getSnapInterval() {
     return getWidth();
   }
 
+  private int getAlignmentOffset() {
+    int width = getWidth();
+    int snapInterval = getSnapInterval();
+    if (mSnapAlignment.equals("center")) {
+      return (width - snapInterval)/2;
+    } else if(mSnapAlignment.equals("end")) {
+      return (width - snapInterval);
+    }
+    return 0;
+  }
+
   public void setEndFillColor(int color) {
     if (color != mEndFillColor) {
       mEndFillColor = color;
@@ -349,7 +368,7 @@ private void smoothScrollToPage(int velocity) {
     if (predictedX > page * width + width / 2) {
       page = page + 1;
     }
-    smoothScrollTo(page * width, getScrollY());
+    smoothScrollTo(page * width - getAlignmentOffset(), getScrollY());
   }
 
   @Override

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

@@ -80,6 +80,11 @@ public void setSnapToInterval(ReactHorizontalScrollView view, float snapToInterv
     view.setSnapInterval((int) (snapToInterval * screenDisplayMetrics.density));
   }
 
+  @ReactProp(name = "snapToAlignment")
+  public void setSnapToAlignment(ReactHorizontalScrollView view, String snapToAlignment) {
+    view.setSnapAlignment(snapToAlignment);
+  }
+
   @ReactProp(name = ReactClippingViewGroupHelper.PROP_REMOVE_CLIPPED_SUBVIEWS)
   public void setRemoveClippedSubviews(ReactHorizontalScrollView view, boolean removeClippedSubviews) {
     view.setRemoveClippedSubviews(removeClippedSubviews);