Pass disabled prop down to native implementation

Summary:
Changelog:
[iOS] [Fixed] - Slider is now disabled properly on iOS if the disabled prop is set.

Reviewed By: yungsters

Differential Revision: D18758835

fbshipit-source-id: 4850ebf05380f241d49d40107de61fd840049779

Libraries/Components/Slider/Slider.js

@@ -237,7 +237,9 @@ const Slider = (
   return (
     <SliderNativeComponent
       {...localProps}
+      // TODO: Reconcile these across the two platforms.
       enabled={!disabled}
+      disabled={disabled}
       maximumValue={maximumValue}
       minimumValue={minimumValue}
       onChange={onChangeEvent}