Make StyleSheet.create type private

Summary:
This type is being used in many places where a much simpler type is often better. In a real pinch this type can still be accessed as so:

```
function returnsStyleSheet(
): $Call<typeof StyleSheet.create, *> {
  return StyleSheet.create({
    root: {
      background: 'white',
    }
  })
}

returnsStyleSheet().foo // foo doesn't exist
returnsStyleSheet().root // okay
```

Reviewed By: yungsters

Differential Revision: D7178524

fbshipit-source-id: 3c0ed03486ca00f1e287261e402fd47807f1fc3d

RNTester/js/RNTesterExampleList.js

@@ -20,15 +20,9 @@ const RNTesterActions = require('./RNTesterActions');
 const RNTesterStatePersister = require('./RNTesterStatePersister');
 const View = require('View');
 
-import type {
-  RNTesterExample,
-} from './RNTesterList.ios';
-import type {
-  PassProps,
-} from './RNTesterStatePersister';
-import type {
-  StyleObj,
-} from 'StyleSheetTypes';
+import type {RNTesterExample} from './RNTesterList.ios';
+import type {PassProps} from './RNTesterStatePersister';
+import type {StyleObj} from 'StyleSheetTypes';
 
 type Props = {
   onNavigate: Function,