BHV-12395: Fix issue with video duration not displaying

Issue
-------
The video duration appears as 00:00 in `moon.VideoPlayer`.

Fix
----
This was due to a name-change of the `duration` property in `moon.VideoPlayer` from `_duration`. `moon.VideoTransportSlider` was originally questionably reaching for a private property of `moon.VideoPlayer`, so this has now been updated to utilize the public property. The private `_currentTime` property is still being retrieved, so this is probably future work involving a non-trivial update to `moon.VideoPlayer`.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>