nuplayer: Add cached lookup of track duration

 * After a seek in an HTTP source (such as Play Music), the duration
   ends up as zero due to multiple pending fetches.
 * This has a nasty side effect of breaking audio effects connected
   to the global mix (visualizer) after an offload fallback as the
   deep buffer stream wasn't selected either.
 * Cache the last known good duration and use it if the extractor
   isn't able to get it.

Change-Id: I65f60323959455ec1ce45ae88b821d4227eeb14d