[0.6.116] use nsec unit instead of msec

[0.6.116] use nsec unit instead of msec

- nsec API will be added for set/get position and get duration
- if libmm-player use nsec as a base time unit,
  it can cover msec and nsec without adding new internal mm api.
- remove deadcode about _adjust_video_position
- remove needless _get_position code in select_track

Change-Id: I74d250be365ba8e6a75f5cc100add3d8cd348a07