BHV-15858: Explicitly show slider knob when slider is re-enabled.

### Issue
We were relying on `moon.Slider`'s `disabledChanged` method to handle showing and hiding of the status knob. This method only handles hiding of the status knob, necessarily, as he default behavior is to only show the status knob when dragging.

### Fix
`moon.VideoPlayer` forces the status knob to appear even when not dragging, so we add an explicit call to show the status knob when the slider is re-enabled.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>