Implement fading edges for ScrollView and it's dependent FlatList (#26163)

Summary:
This should add props for enabling horizontal and vertical fading
edges for Scrollview and FlatList.
These fading edges are used to communicate to the user that there is more content to see.

## Changelog

[Android] [Added] - fading edges props to the FlatList and ScrollView components
Pull Request resolved: https://github.com/facebook/react-native/pull/26163

Test Plan:
Open the React Native test app and navigate to the FlatList section.
Enable the `useFadingEdges` switch and insert a number into `Fading edge length`.

![device-2019-08-23-123745](https://user-images.githubusercontent.com/222393/63587150-7385cb00-c5a3-11e9-98dc-bffe8276d30c.png)
![device-2019-08-23-123844](https://user-images.githubusercontent.com/222393/63587156-75e82500-c5a3-11e9-9e9f-66876ac8f506.png)

Differential Revision: D16992488

Pulled By: sahrens

fbshipit-source-id: f72a9a890d9056bb017cc5747c6f66b7c35633dd