Navigation Back support and examples for Android

Summary:
public
- Intro new back action
- Add support in the two main reducers
- Use it in examples to support Android back button
- Disable NavigationCard gestures on Android

Reviewed By: hedgerwang

Differential Revision: D2914154

fb-gh-sync-id: d4dce6538e19613a2ffca21e2e3b2ecaded3d5dc
shipit-source-id: d4dce6538e19613a2ffca21e2e3b2ecaded3d5dc

Examples/UIExplorer/NavigationExperimental/NavigationTabsExample.js

@@ -64,12 +64,13 @@ const ExampleTabsReducer = NavigationReducer.TabsReducer({
   ],
 });
 
-const NavigationTabsExample = React.createClass({
-  render: function() {
+class NavigationTabsExample extends React.Component {
+  render() {
     return (
       <NavigationRootContainer
         reducer={ExampleTabsReducer}
         persistenceKey="NAV_EXAMPLE_STATE_TABS"
+        ref={navRootContainer => { this.navRootContainer = navRootContainer; }}
         renderNavigation={(navigationState) => {
           if (!navigationState) { return null; }
           return (
@@ -88,8 +89,14 @@ const NavigationTabsExample = React.createClass({
         }}
       />
     );
-  },
-});
+  }
+  handleBackAction() {
+    return (
+      this.navRootContainer &&
+      this.navRootContainer.handleNavigation(NavigationRootContainer.getBackAction())
+    );
+  }
+}
 
 const styles = StyleSheet.create({
   topView: {