fix setNativeSelectedPosition native command

Summary:
telling native that `selected` is the new selection is basically ignoring the most recent user input and resetting it back to the previous value. the reason it flips back and forth is that by the next time `onSelect` is called, the above code has updated the value of `selected` (line 63)

Changelog:
[Android] [Fixed] Picker - fix usage of setNativeSelectedPosition in onSelect

Reviewed By: mdvacca

Differential Revision: D22022456

fbshipit-source-id: fe51f9cbd712b5578e9fd9ea2992bd12d48f4ec4

Libraries/Components/Picker/PickerAndroid.android.js

@@ -100,7 +100,7 @@ function PickerAndroid(props: Props): React.Node {
           props.mode === 'dropdown'
             ? AndroidDropdownPickerCommands
             : AndroidDialogPickerCommands;
-        Commands.setNativeSelectedPosition(current, selected);
+        Commands.setNativeSelectedPosition(current, position);
       }
     },
     [