Add snapToInterval support for Android ScrollView

[sigvef]

This pull request adds Android support for snapToInterval in the ScrollView component (it is already implemented for iOS). I need it for a project, so I will be responsive and follow up on any comments on this pull request :)

Test Plan

The code change is pretty minimal, so it should be easy to reason about. I made a quick bare-bones test project with react-native init and tested it in an android emulator.

The test project code:

export default class ScrollViewTest extends Component {
render() {
const width = Dimensions.get('window').width - 32;
return (
<ScrollView
horizontal={true}
pagingEnabled={true}
snapToInterval={width}
>
<Text style={{width, backgroundColor: 'red'}}>
Welcome to React Native!
</Text>
<Text style={{width, backgroundColor: 'green'}}>
To get started, edit index.android.js
</Text>
<Text style={{width, backgroundColor: 'blue'}}>
Double tap R on your keyboard to reload,{'\n'}
Shake or press menu button for dev menu
</Text>
</ScrollView>
);
}
}

A video of it running:

[sigvef]

(fwiw, the builds are failing because master is red atm)

[henrikra]

I have already done PR about this #15297

[sigvef]

Nice! Hope this means that we can get this feature soon :)

[facebook-github-bot]

@sigvef I tried to find reviewers for this pull request and wanted to ping them to take another look. However, based on the blame information for the files in this pull request I couldn't find any reviewers. This sometimes happens when the files in the pull request are new or don't exist on master anymore. Is this pull request still relevant? If yes could you please rebase? In case you know who has context on this code feel free to mention them in a comment (one person is fine). Thanks for reading and hope you will continue contributing to the project.

[ghost]

Hey! I'm willing to do anything at all to help along this PR from August being merged before 2017 is gone forever and 2018 rolls around. Please let me know if there's anything I can do to help.

[sigvef]

This feature has now been implemented in ddd65f1, and landed in RN 0.53.