[ReactNative] Navigator initialRoute default change

Summary:
The default initialRoute was the first route, but it makes more sense for the default initialRoute to be the last route in the initialRouteStack.

Updated the docs to reflect that.

@public

Test Plan: Updated call sites and checked that they work. Not many places use initialRouteStack yet.

Libraries/CustomComponents/Navigator/Navigator.js

@@ -227,15 +227,17 @@ var Navigator = React.createClass({
     renderScene: PropTypes.func.isRequired,
 
     /**
-     * Provide a single "route" to start on. A route is an arbitrary object
+     * Specify a route to start on. A route is an object that the navigator
-     * that the navigator will use to identify each scene before rendering.
+     * will use to identify each scene to render. `initialRoute` must be
-     * Either initialRoute or initialRouteStack is required.
+     * a route in the `initialRouteStack` if both props are provided. The
+     * `initialRoute` will default to the last item in the `initialRouteStack`.
      */
     initialRoute: PropTypes.object,
 
     /**
-     * Provide a set of routes to initially mount the scenes for. Required if no
+     * Provide a set of routes to initially mount. Required if no initialRoute
-     * initialRoute is provided
+     * is provided. Otherwise, it will default to an array containing only the
+     * `initialRoute`
      */
     initialRouteStack: PropTypes.arrayOf(PropTypes.object),
 
@@ -295,21 +297,18 @@ var Navigator = React.createClass({
   },
 
   getInitialState: function() {
-    var routeStack = this.props.initialRouteStack || [];
+    var routeStack = this.props.initialRouteStack || [this.props.initialRoute];
-    var initialRouteIndex = 0;
+    invariant(
-    if (this.props.initialRoute && routeStack.length) {
+      routeStack.length >= 1,
+      'Navigator requires props.initialRoute or props.initialRouteStack.'
+    );
+    var initialRouteIndex = routeStack.length - 1;
+    if (this.props.initialRoute) {
       initialRouteIndex = routeStack.indexOf(this.props.initialRoute);
       invariant(
         initialRouteIndex !== -1,
         'initialRoute is not in initialRouteStack.'
       );
-    } else if (this.props.initialRoute) {
-      routeStack = [this.props.initialRoute];
-    } else {
-      invariant(
-        routeStack.length >= 1,
-        'Navigator requires props.initialRoute or props.initialRouteStack.'
-      );
     }
     return {
       sceneConfigStack: routeStack.map(