Finish AccessibilityInfo implementation

Summary:
This PR is based on files ericvicenti gave me. Specifically, he gave me:
  - AccessibilityInfo.android.js
  - AccessibilityInfo.ios.js
  - AccessibilityInfoModule.java

Before this change, only a native iOS implementation of AccessibilityInfo existed. This change includes:
  - A native Android implementation of AccessibilityInfo.
  - JavaScript wrappers for the AccessibilityInfo module for both iOS and Android.
  - UIExplorer changes to illustrate how to use AccessibilityInfo on iOS and Android.
  - Documentation for the AccessibilityInfo APIs.

**Test plan (required)**

Tested the UIExplorer AccessibilityInfo example on iOS and Android with the screen reader both enabled and disabled.

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/12273

Reviewed By: mkonicek

Differential Revision: D4527224

Pulled By: ericvicenti

fbshipit-source-id: d04638465ccbdbb35ecfc9504daaeb8e33aab57a

Examples/UIExplorer/js/AccessibilityAndroidExample.android.js

@@ -25,6 +25,7 @@
 var React = require('react');
 var ReactNative = require('react-native');
 var {
+  AccessibilityInfo,
   StyleSheet,
   Text,
   View,
@@ -45,8 +46,34 @@ class AccessibilityAndroidExample extends React.Component {
     count: 0,
     backgroundImportantForAcc: 0,
     forgroundImportantForAcc: 0,
+    screenReaderEnabled: false,
   };
 
+  componentDidMount() {
+    AccessibilityInfo.addEventListener(
+      'change',
+      this._handleScreenReaderToggled
+    );
+    AccessibilityInfo.fetch().done((isEnabled) => {
+      this.setState({
+        screenReaderEnabled: isEnabled
+      });
+    });
+  }
+
+  componentWillUnmount() {
+    AccessibilityInfo.removeEventListener(
+      'change',
+      this._handleScreenReaderToggled
+    );
+  }
+
+  _handleScreenReaderToggled = (isEnabled) => {
+    this.setState({
+      screenReaderEnabled: isEnabled,
+    });
+  }
+
   _addOne = () => {
     this.setState({
       count: ++this.state.count,
@@ -125,6 +152,12 @@ class AccessibilityAndroidExample extends React.Component {
           </Text>
         </UIExplorerBlock>
 
+        <UIExplorerBlock title="Check if the screen reader is enabled">
+          <Text>
+            The screen reader is {this.state.screenReaderEnabled ? 'enabled' : 'disabled'}.
+          </Text>
+        </UIExplorerBlock>
+
         <UIExplorerBlock title="Overlapping views and importantForAccessibility property">
           <View style={styles.container}>
             <View