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/NavigationBasicExample.js

@@ -40,6 +40,7 @@ const NavigationBasicExample = React.createClass({
       <NavigationRootContainer
         reducer={NavigationBasicReducer}
         persistenceKey="NavigationBasicExampleState"
+        ref={navRootContainer => { this.navRootContainer = navRootContainer; }}
         renderNavigation={(navState, onNavigate) => {
           if (!navState) { return null; }
           return (
@@ -69,6 +70,14 @@ const NavigationBasicExample = React.createClass({
       />
     );
   },
+
+  handleBackAction: function() {
+    return (
+      this.navRootContainer &&
+      this.navRootContainer.handleNavigation(NavigationRootContainer.getBackAction())
+    );
+  },
+
 });
 
 const styles = StyleSheet.create({