Introduce DeviceInfo as a new native module

Summary:
The `UIManager` already has a lot of responsibilities and is deeply
tied with React Native's view architecture. This diff separates out a
`DeviceInfo` native module to provide information about screen dimensions and
font scale, etc.

Reviewed By: fkgozali

Differential Revision: D4713834

fbshipit-source-id: f2ee93acf876a4221c29a8c731f5abeffbb97974

Libraries/react-native/react-native-implementation.js

@@ -85,6 +85,7 @@ const ReactNative = {
   get CameraRoll() { return require('CameraRoll'); },
   get Clipboard() { return require('Clipboard'); },
   get DatePickerAndroid() { return require('DatePickerAndroid'); },
+  get DeviceInfo() { return require('DeviceInfo'); },
   get Dimensions() { return require('Dimensions'); },
   get Easing() { return require('Easing'); },
   get I18nManager() { return require('I18nManager'); },