Typing ViewProp, ImageProp

Summary:
Tightening these types.

We can't type TextProp the same way yet because of something fishy in Flow:

https://flow.org/try/#0KYDwDg9gTgLgBDAnmYcAqoYGUkBtUC8cA3gD4BQccAZhAHYwBiAhgLYCWuiAXHAM4wo7OgHMANJRr1s7AF7BedAK6sARsCgTSAXwDc5cqEiwEyVIwAiWRtJyJ8cImUm0GLDl14ChorXoNG0PBIKOiYdvgAClAQYI5wACQASsDMACYA8nRcADwJWAAWzCg5GCDYeMAAfFW6cAD09XAprBAAbqjJqZnZiHDMdGkIBagaMVBwIhDAfP0A7syIBgDG9AJwllgRChtWNgzb8cQu0u6cPHAA5LQQlxL+5Kt062UV9sAAasxQvK-b0bF4pttrogA

Reviewed By: olegbl

Differential Revision: D7165134

fbshipit-source-id: 2fd3de2af8bdc53231d59de71def1e6a7221aff7

Author: elicwhite

Libraries/StyleSheet/StyleSheetTypes.js

@@ -241,9 +241,9 @@ export type StyleProp<+T> =
 export type StyleObj = StyleProp<$Shape<Style>>;
 export type StyleValue = StyleObj;
 
-export type ViewStyleProp = StyleObj;
+export type ViewStyleProp = StyleProp<$ReadOnly<$Shape<ViewStyle>>>;
 export type TextStyleProp = StyleObj;
-export type ImageStyleProp = StyleObj;
+export type ImageStyleProp = StyleProp<$ReadOnly<$Shape<ImageStyle>>>;
 
 export type Styles = {
   +[key: string]: $Shape<Style>,