Summary: In accordance with the unwritten rule that any API that takes a callback should also return a promise, I've changed `InteractionManager.runAfterInteractions()` to do just that.
```js
InteractionManager.runAfterInteractions(() => {
...
});
```
can become
```js
InteractionManager.runAfterInteractions().then(() => {
...
});
```
(but doesn't have to). Most importantly, though, this change enables code like
```js
doSomeUIStuff();
await InteractionManager.runAfterInteractions();
doSomeNonUIStuff();
```
which is nice.
Note: Because returning a `Promise` means the callback argument is now optional, the behaviour of the API is slightly changed, though not in a backwards-incompatible way (unless a consumer is in some way relying on the exception, but that would be insane).
Closes https://github.com/facebook/react-native/pull/3788
Reviewed By: vjeux
Differential Revision: D2634693
Pulled By: josephsavona
fb-gh-sync-id: 7315120963be23cf69d777e940b2750d32ae47a8
d7c8b4478b