Some touches on the NumberPicker UI.

1. Shoing the scroll wheel and fades it out the first time
   the widget is displayed.

2. Added separators for easing use to figure out which will
   be the current selection while scrolling.

3. Updated the fling scroller to user decelerating interpolator.

Change-Id: Ie75519564ace8f44cc6f8eed692182153f839694