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

Libraries/StyleSheet/StyleSheet.js

@@ -19,15 +19,14 @@ const flatten = require('flattenStyle');
 import type {
   StyleSheetStyle as _StyleSheetStyle,
   Styles as _Styles,
-  StyleSheet as _StyleSheet,
   StyleObj,
   LayoutStyle
 } from 'StyleSheetTypes';
 
 export type StyleProp = StyleObj;
 export type Styles = _Styles;
-export type StyleSheet<S> = _StyleSheet<S>;
 export type StyleSheetStyle = _StyleSheetStyle;
+type StyleSheet<+S: Styles> = $ObjMap<S, (Object) => StyleSheetStyle>;
 
 let hairlineWidth = PixelRatio.roundToNearestPixel(0.4);
 if (hairlineWidth === 0) {

Libraries/StyleSheet/StyleSheetTypes.js

@@ -240,8 +240,6 @@ export type Styles = {
   +[key: string]: $Shape<Style>,
 };
 
-export type StyleSheet<+S: Styles> = $ObjMap<S, (Object) => StyleSheetStyle>;
-
 /*
 Utility type get non-nullable types for specific style keys.
 Useful when a component requires values for certain Style Keys.

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,