Added NavigationBackAndroid to public api

Typescript didn't like import('NavigationBackAndroid') because it there
isn't a NavigationBackAndroid.ts
(microsoft/TypeScript#8328). The workarounds
in the issue don't seem right so just used require directly instead of
import. Need to import specific files so that they appear in the built
package

NavigationReactNative/src/NavigationBackAndroid.ios.ts

@@ -1,5 +1 @@
-import * as React from 'react';
-
-var NavigationBackAndroid = () => null;
-
-export default NavigationBackAndroid;
+export default () => null;

NavigationReactNative/src/NavigationReactNative.ts

@@ -1,4 +1,7 @@
+import NavigationBackAndroid_android from './NavigationBackAndroid.android';
+import NavigationBackAndroid_ios from './NavigationBackAndroid.ios';
+var NavigationBackAndroid = require('./NavigationBackAndroid');
 import SceneNavigator from './SceneNavigator';
 import spring from './spring';
 
-export { SceneNavigator, spring };
+export { NavigationBackAndroid, SceneNavigator, spring };