message: Implemented activity navigation
diff: package com.navigation.reactnative;
  import android.app.Activity;
  +import android.content.Intent;
  class ActivityNavigator extends SceneNavigator {
  @Override
  void navigateBack(int currentCrumb, int crumb, Activity activity, NavigationStackView stack) {
  -
  + Intent intent = new Intent(activity, SceneActivity.class);
  + String key = stack.keys.getString(crumb);
  + intent.putExtra(SceneActivity.KEY, key);
  + intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
  + int enter = getAnimationResourceId(activity, stack.enterAnim, stack.activityCloseEnterAnimationId);
  + int exit = getAnimationResourceId(activity, stack.exitAnim, stack.activityCloseExitAnimationId);
  + activity.navigateUpTo(intent);
  + activity.overridePendingTransition(enter, exit);
  }
  @Override
  void navigate(int currentCrumb, int crumb, Activity activity, NavigationStackView stack) {
  -
  + Intent[] intents = new Intent[crumb - currentCrumb];
  + for(int i = 0; i < crumb - currentCrumb; i++) {
  + int nextCrumb = currentCrumb + i + 1;
  + Intent intent = new Intent(activity, SceneActivity.class);
  + String key = stack.keys.getString(nextCrumb);
  + intent.putExtra(SceneActivity.KEY, key);
  + intents[i] = intent;
  + }
  + int enter = getAnimationResourceId(activity, stack.enterAnim, stack.activityOpenEnterAnimationId);
  + int exit = getAnimationResourceId(activity, stack.exitAnim, stack.activityOpenExitAnimationId);
  + activity.startActivities(intents);
  + activity.overridePendingTransition(enter, exit);
  }
  @Override
  void refresh(int currentCrumb, int crumb, Activity activity, NavigationStackView stack) {
  -
  + Intent intent = new Intent(activity, SceneActivity.class);
  + String key = stack.keys.getString(crumb);
  + intent.putExtra(SceneActivity.KEY, key);
  + int enter = getAnimationResourceId(activity, stack.enterAnim, stack.activityOpenEnterAnimationId);
  + int exit = getAnimationResourceId(activity, stack.exitAnim, stack.activityOpenExitAnimationId);
  + activity.finish();
  + activity.startActivity(intent);
  + activity.overridePendingTransition(enter, exit);
  }
  }
  