Emit willfocus from immediatelyResetRouteStack

Summary:
The Navigator component inconsistently emits the `willfocus` event.

While it is emitted in `Navigator#resetTo`, `Navigator#push`, `Navigator#pop` and event `Navigator#replaceAtIndex` it is not emitted from `Navigator#immediatelyResetRouteStack`. This leads to surprising inconsistencies when working with these events.

With the PR I suggest to emit the `willfocus` event before resetting the route stack.
Closes #10125

Differential Revision: D3931284

fbshipit-source-id: 6e4f45c6d38426bcd0acc8f8c39478524032a03a

Libraries/CustomComponents/Navigator/Navigator.js

@@ -484,6 +484,7 @@ var Navigator = React.createClass({
    */
   immediatelyResetRouteStack: function(nextRouteStack) {
     var destIndex = nextRouteStack.length - 1;
+    this._emitWillFocus(nextRouteStack[destIndex]);
     this.setState({
       routeStack: nextRouteStack,
       sceneConfigStack: nextRouteStack.map(