[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/Navigation/NavigationEvent.js

@@ -0,0 +1,21 @@
+/**
+ * Copyright 2004-present Facebook. All Rights Reserved.
+ *
+ * @providesModule NavigationEvent
+ * @flow
+ */
+'use strict';
+
+class NavigationEvent {
+  type: String;
+  target: Object;
+  data: any;
+
+  constructor(type: String, target: Object, data: any) {
+    this.type = type;
+    this.target = target;
+    this.data = data;
+  }
+}
+
+module.exports = NavigationEvent;