Android: Add overScrollMode prop to ScrollView

Summary:
This prop exposes the functionality provided by Android ScrollView's setOverScrollMode method.

One interesting thing to note is that, if you were to read the Android docs, you would think that the value "always" is the default over scroll mode. However, the docs are incorrect and "always-if-content-scrolls" is actually the default value (http://stackoverflow.com/a/27116306).

**Test plan (required)**

Verified this change in a test app. Also, our team uses this change in our app.

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/10905

Differential Revision: D4500957

Pulled By: mkonicek

fbshipit-source-id: 873eba38183defba133c228e0c1038efa83297d3

Libraries/Components/ScrollView/ScrollView.js

@@ -349,6 +349,24 @@ const ScrollView = React.createClass({
      * @platform android
      */
     scrollPerfTag: PropTypes.string,
+
+     /**
+     * Used to override default value of overScroll mode.
+     *
+     * Possible values:
+     *
+     *  - `'auto'` - Default value, allow a user to over-scroll
+     *    this view only if the content is large enough to meaningfully scroll.
+     *  - `'always'` - Always allow a user to over-scroll this view.
+     *  - `'never'` - Never allow a user to over-scroll this view.
+     *
+     * @platform android
+     */
+    overScrollMode: PropTypes.oneOf([
+      'auto',
+      'always',
+      'never',
+    ]),
   },
 
   mixins: [ScrollResponder.Mixin],