Add VideoStillView

If no video still is present in the video's properties, a black scrim
will be presented instead.

This serves two purposes:

1. If a `kBCOVCatalogJSONKeyVideoStillUrl` is present in the BCOVVideo's
   properties, it will be used to present a still image overlay on top of the player
   until playback begins, at which time it will be dismissed permanently.
2. The Google Ads SDK for IMA exhibits a slight delay before presenting
   any preroll ads, which can be unsightly in most videos (it means the
   first frame of the video is briefly visible before the preroll
   begins). By presenting the video still or a black scrim, this
   unsightliness is obscured.

(As with all Brightcove sample code, this example is provided as-is, for educational purposes only, with no warranties implied, etc.)