Summary:
- Properly inherit flow types from base components, including `defaultProps`
- template-ify the `Item` type as it flows from the `data` prop into `ItemComponent`
Note that for `SectionList` is is harder to do the `Item` typing because each section in the
`sections` array can have a different `Item` type, plus all the optional overrides...not sure how to
tackle that.
Reviewed By: yungsters
Differential Revision: D4557523
fbshipit-source-id: a0c5279fcdaabe6aab5fe11743a99c3715a44032
Spencer Ahrens