Add default Instance type and variance to AnimatedComponentType
Add default Instance type and variance to AnimatedComponentType

Summary:
Updates `AnimatedComponentType`'s type arguments to match [`React.AbstractComponent`](https://flow.org/en/docs/react/types/#toc-react-abstractcomponent)'s more closely in [variance](https://flow.org/en/docs/types/generics/#toc-variance-sigils) and defaults (`Instance = mixed`).

Changelog: [Internal]

Reviewed By: genkikondo

Differential Revision: D34254030

fbshipit-source-id: 622d1e4ecbc493bba8ec857454dd28583b677d78

janicduplessis/react-native,janicduplessis/react-native,facebook/react-native,facebook/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,facebook/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,facebook/react-native