BREAKING: Android: Correct value of Dimensions.get('screen').fontScale

Summary:
The PR description has been updated to reflect the new approach.

**Breaking Change Summary**

On Android, the following properties now return a different number:
  - `Dimensions.get('window').fontScale`
  - `Dimensions.get('screen').fontScale`
  - `PixelRatio.getFontScale()`

This is a breaking change to anyone who was using these properties because the meaning of these properties has now changed.

These properties used to return a value representing font scale times density ([`DisplayMetrics.scaledDensity`](https://developer.android.com/reference/android/util/DisplayMetrics.html#scaledDensity)). Now they return a value representing just font scale ([`Configuration.fontScale`](https://developer.android.com/reference/android/content/res/Configuration.html#fontScale)).

**PR Description**

This PR changes a few things:
  - Correctly exposes the font scale to JavaScript as `Dimensions.get('screen').fontScale`. UIManager was exporting `DisplayMetrics.scaledDensity` under the name `fontScale`. How
Closes https://github.com/facebook/react-native/pull/11008

Differential Revision: D4558207

Pulled By: astreet

fbshipit-source-id: 096ce7b28051325dfd45fdb2a14b5e9b7d3bc46f

Libraries/Utilities/Dimensions.js

@@ -11,12 +11,15 @@
  */
 'use strict';
 
+var EventEmitter = require('EventEmitter');
 var Platform = require('Platform');
 var UIManager = require('UIManager');
 var RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
 
 var invariant = require('fbjs/lib/invariant');
 
+var eventEmitter = new EventEmitter();
+var dimensionsInitialized = false;
 var dimensions = {};
 class Dimensions {
   /**
@@ -60,6 +63,15 @@ class Dimensions {
     }
 
     Object.assign(dimensions, dims);
+    if (dimensionsInitialized) {
+      // Don't fire 'change' the first time the dimensions are set.
+      eventEmitter.emit('change', {
+        window: dimensions.window,
+        screen: dimensions.screen
+      });
+    } else {
+      dimensionsInitialized = true;
+    }
   }
 
   /**
@@ -81,6 +93,39 @@ class Dimensions {
     invariant(dimensions[dim], 'No dimension set for key ' + dim);
     return dimensions[dim];
   }
+
+  /**
+   * Add an event handler. Supported events:
+   *
+   * - `change`: Fires when a property within the `Dimensions` object changes. The argument
+   *   to the event handler is an object with `window` and `screen` properties whose values
+   *   are the same as the return values of `Dimensions.get('window')` and
+   *   `Dimensions.get('screen')`, respectively.
+   */
+  static addEventListener(
+    type: string,
+    handler: Function
+  ) {
+    invariant(
+      'change' === type,
+      'Trying to subscribe to unknown event: "%s"', type
+    );
+    eventEmitter.addListener(type, handler);
+  }
+
+  /**
+   * Remove an event handler.
+   */
+  static removeEventListener(
+    type: string,
+    handler: Function
+  ) {
+    invariant(
+      'change' === type,
+      'Trying to remove listener for unknown event: "%s"', type
+    );
+    eventEmitter.removeListener(type, handler);
+  }
 }
 
 Dimensions.set(UIManager.Dimensions);