Add position to Picker onValueChange's call

Summary:According to the [docs](http://facebook.github.io/react-native/releases/0.20/docs/picker.html#onvaluechange), `Picker`'s `onValueChange` should be called with `itemValue` and `itemPosition` but currently only `itemValue` is passed. This PR fixes that.
The position is passed as the 2nd parameter to not introduce any breaking change, and also to respect the current documentation.
The documentation doesn't need to be changed as it will be correct with this PR, but maybe it needs to be updated until this is merged?
Closes #5874

Differential Revision: D2953936

Pulled By: nicklockwood

fb-gh-sync-id: b8c1283013d4c7ed315066d8750f601f76f6bbb2
shipit-source-id: b8c1283013d4c7ed315066d8750f601f76f6bbb2

Libraries/Components/Picker/PickerAndroid.android.js

@@ -100,9 +100,9 @@ var PickerAndroid = React.createClass({
       var position = event.nativeEvent.position;
       if (position >= 0) {
         var value = this.props.children[position].props.value;
-        this.props.onValueChange(value);
+        this.props.onValueChange(value, position);
       } else {
-        this.props.onValueChange(null);
+        this.props.onValueChange(null, position);
       }
     }
 

Libraries/Picker/PickerIOS.ios.js

@@ -54,7 +54,7 @@ var PickerIOS = React.createClass({
     });
     return {selectedIndex, items};
   },
-  
+
   render: function() {
     return (
       <View style={this.props.style}>
@@ -74,7 +74,7 @@ var PickerIOS = React.createClass({
       this.props.onChange(event);
     }
     if (this.props.onValueChange) {
-      this.props.onValueChange(event.nativeEvent.newValue);
+      this.props.onValueChange(event.nativeEvent.newValue, event.nativeEvent.newIndex);
     }
 
     // The picker is a controlled component. This means we expect the