Expose constants to Js from Android

Currently exporting `backButton` id, usage: `Navigation.constants.backButton`

lib/android/app/src/main/java/com/reactnativenavigation/react/Constants.java

 package com.reactnativenavigation.react;
 
 public class Constants {
+    public static final String BACK_BUTTON_JS_KEY = "backButton";
     public static final String BACK_BUTTON_ID = "RNN.back";
 }

lib/android/app/src/main/java/com/reactnativenavigation/react/NavigationModule.java

 
 import org.json.JSONObject;
 
+import java.util.HashMap;
 import java.util.Map;
 
 public class NavigationModule extends ReactContextBaseJavaModule {
 		return NAME;
 	}
 
+	@Override
+    public Map<String, Object> getConstants() {
+        final Map<String, Object> constants = new HashMap<>();
+        constants.put(Constants.BACK_BUTTON_JS_KEY, Constants.BACK_BUTTON_ID);
+        return constants;
+    }
+
 	@ReactMethod
 	public void setRoot(String commandId, ReadableMap rawLayoutTree, Promise promise) {
 		final LayoutNode layoutTree = LayoutNodeParser.parse(new JSONObject(rawLayoutTree.toHashMap()));

lib/src/Navigation.ts

 import { Element } from './adapters/Element';
 import { ComponentEventsObserver } from './events/ComponentEventsObserver';
 import { CommandsObserver } from './events/CommandsObserver';
+import { Constants } from './constants/Constants';
+import { NativeModules } from 'react-native';
+
+const NavigationModule = NativeModules.RNNBridgeModule;
 
 export class Navigation {
   public readonly Element: React.ComponentType<{ elementId: any; resizeMode: any; }>;
+  public readonly constants = new Constants(NavigationModule);
 
   private readonly store;
   private readonly nativeEventsReceiver;

lib/src/constants/Constants.test.ts

+import { Constants } from './Constants';
+
+const NavigationModule = {
+  backButton: 'backButton'
+};
+
+describe('Constants', () => {
+  let uut: Constants;
+
+  beforeEach(() => {
+    uut = new Constants(NavigationModule);
+  });
+
+  it('backButton', () => {
+    expect(uut.backButton).toEqual(NavigationModule.backButton);
+  });
+});

lib/src/constants/Constants.ts

+export class Constants {
+  public readonly backButton;
+
+  constructor(navigationModule) {
+    this.backButton = navigationModule.backButton;
+  }
+}