[Navigator]: Allow developer to observe the focus change events from the owner or the children

of the navigator component.

Summary:
Per offline discussion with @evv, we'd like to deprecate the `onDidFocus` and `onWillFocus`
API that makes it really hard for the descendent children of a navigator to observe its focus
change events.
@public

Since for now the descendent children do have access to the navigator via `this.props.navigator`,
this diff makes it easy to observe the focus change event by doing:

```
this.props.navigator.addListener('willfocus', this._onFocus);
```

The goal is to make the event system in navigator more useful and maintainable.

Test Plan:
Test Video: https://www.facebook.com/pxlcld/mrzS
1. jest:  ./Libraries/FBReactKit/js/runTests.js NavigationEventEmitter
2. Load UI Explorer: <Navigator />, see console logs that shows the focus change events fires.

Libraries/CustomComponents/Navigator/Navigator.js

@@ -30,11 +30,11 @@
 var AnimationsDebugModule = require('NativeModules').AnimationsDebugModule;
 var Dimensions = require('Dimensions');
 var InteractionMixin = require('InteractionMixin');
+var NavigationContext = require('NavigationContext');
 var NavigatorBreadcrumbNavigationBar = require('NavigatorBreadcrumbNavigationBar');
 var NavigatorNavigationBar = require('NavigatorNavigationBar');
 var NavigatorSceneConfigs = require('NavigatorSceneConfigs');
 var PanResponder = require('PanResponder');
-var Platform = require('Platform');
 var React = require('React');
 var StaticContainer = require('StaticContainer.react');
 var StyleSheet = require('StyleSheet');
@@ -203,11 +203,17 @@ var Navigator = React.createClass({
     initialRouteStack: PropTypes.arrayOf(PropTypes.object),
 
     /**
+     * @deprecated
+     * Use `navigationContext.addListener('willfocus', callback)` instead.
+     *
      * Will emit the target route upon mounting and before each nav transition
      */
     onWillFocus: PropTypes.func,
 
     /**
+     * @deprecated
+     * Use `navigationContext.addListener('didfocus', callback)` instead.
+     *
      * Will be called with the new route of each scene after the transition is
      * complete or after the initial mounting
      */
@@ -321,7 +327,10 @@ var Navigator = React.createClass({
   },
 
   componentWillUnmount: function() {
-
+    if (this._navigationContext) {
+      this._navigationContext.dispose();
+      this._navigationContext = null;
+    }
   },
 
   /**
@@ -461,12 +470,16 @@ var Navigator = React.createClass({
   },
 
   _emitDidFocus: function(route) {
+    this.navigationContext.emit('didfocus', {route: route});
+
     if (this.props.onDidFocus) {
       this.props.onDidFocus(route);
     }
   },
 
   _emitWillFocus: function(route) {
+    this.navigationContext.emit('willfocus', {route: route});
+
     var navBar = this._navBar;
     if (navBar && navBar.handleWillFocus) {
       navBar.handleWillFocus(route);
@@ -1139,6 +1152,14 @@ var Navigator = React.createClass({
       </View>
     );
   },
+
+  // Getter for `navigationContext`.
+  get navigationContext() {
+    if (!this._navigationContext) {
+      this._navigationContext = new NavigationContext();
+    }
+    return this._navigationContext;
+  }
 });
 
 module.exports = Navigator;