Added persistentScrollbar prop in ScrollView component (#22300)

Summary:
This PR is a follow-up from the closed [discussions-and-proposals PR](https://github.com/react-native-community/discussions-and-proposals/pull/39), that explains in more detail the rationale for adding this feature.
Pull Request resolved: https://github.com/facebook/react-native/pull/22300

Reviewed By: fkgozali

Differential Revision: D13121748

Pulled By: mdvacca

fbshipit-source-id: 899641be79bdb41fa6649df0772c602a5e09b3b9

Libraries/Components/ScrollView/ScrollView.js

@@ -301,6 +301,13 @@ type AndroidProps = $ReadOnly<{|
    * @platform android
    */
   overScrollMode?: ?('auto' | 'always' | 'never'),
+  /**
+   * Causes the scrollbars not to turn transparent when they are not in use.
+   * The default value is false.
+   *
+   * @platform android
+   */
+  persistentScrollbar?: ?boolean,
 |}>;
 
 type VRProps = $ReadOnly<{|
@@ -442,6 +449,7 @@ export type Props = $ReadOnly<{|
    * Note: Vertical pagination is not supported on Android.
    */
   pagingEnabled?: ?boolean,
+
   /**
    * When false, the view cannot be scrolled via touch interaction.
    * The default value is true.