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, from `moon.VideoPlayer`. This method only handles hiding of the status knob, necessarily, as the 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>