Rename backButton id constant to backButtonId

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_JS_KEY = "backButtonId";
     public static final String BACK_BUTTON_ID = "RNN.back";
     public static final String STATUS_BAR_HEIGHT_KEY = "statusBarHeight";
 }