nuplayer: Seek to valid position in setVideoSurface

- If flush is called on both A/V tracks and video
  decoder is not yet created, In offload case the
  first audio buffer will get flush and audio playback
  starts from timestamp of second buffer, where as
  video playback starts from zero leads to AV sync
  issue in start and cause framedrops.

- Call seek to current position to align the Audio
  and Video tracks. Seek to zero if playback has not
  yet started.

Change-Id: I4f75d0aa908714ad292f348fa8bf7aa5f837f8d7