measure and layout after adding a view to the container

[danilobuerger]

Imported from facebook/react-native#18609

Currently, when adding views to ViewPagerAndroid after it was attached, those views would not show up as they had a size of 0. This fixes that by measuring and layouting after adding a view to the container.

Fixes facebook/react-native#14296

Test Plan

See https://github.com/danilobuerger/react-native-viewpagerandroid-dynamic

1. yarn install
2. yarn run android
3. You will see two pages
4. Press "ADDPAGE"
5. Scroll to the third page, it will be empty
6. Use react-native master with this patch
7. yarn run android
8. You will see two pages
9. Press "ADDPAGE"
10. Scroll to the third page, it will show "3 page (Scroll Me)"

[satya164]

Thanks ❤️

[cpojer]

This looks good to me. Please do make a release of ViewPager on npm before merging this PR though.

[ferrannp]

Awesome! Will merge this after making the first release. After that, we can add your Add Page button to the example here too :)

[danilobuerger]

@ferrannp Thanks! Could you also cut a new release with this fix please? I guess this repo won't have much activity for the first few weeks until people start migrating, so it would be nice if its already there.

[ferrannp]

Yup, thank you!

[danilobuerger]

@ferrannp Awesome, thanks!