NuPlayer: Don't maintain timeStamp if state is running

  While offload playback is going on, if user seeks
  to new position and after playback for some time,
  pauses till offload tear-down happens then playback
  resumes with old seek position

  The book-keeping of seektime for start of playback
  mStartupSeekTimeUs is also done in running state,
  it should be done only if current state is paused.

Change-Id: I4173a2ee4244bcc6794822dde24c467f0189b84f