nuplayer: ensure using video ts as anchor when really has no

audio and clearAnchor must be followed by updateAnchor in video only case

For video only clips AnchorTime is zero since it is updated only in the start. Due to this there is a delay in rendering the buffers. Video should clear and update anchor at the same time  while playback rate change. Because video postDrainVideoQueue set message kWhatDrainVideoQueue into MediaClock timer, if anchor in MediaClock is cleared, timer post message will be blocked. This will cause video stuck. So, in video only case, clearAnchor must be followed by updateAnchor immediately

Author: JiaJia Cong

Bug: 263545819

Change-Id: I58a3835aa08dfb57faf791103c8b9add0a6c502f