[0.6.106] fix state transition issue

[0.6.106] fix state transition issue

- there is state transition error during buffering.
- player_start
  -> buffering start(async pause) : not finished
     state : (p)paused, (c)playing, (p)paused, (t)playing
  -> buffering stop(async resume) : not finished
     state : (p)paused, (c)PLAYING, (p)PLAYING, (t)playing
  -> player_pause

  if there is a new pause request in case the buffering handling is not completed
  (== current : playing, pending : playing)
  the request will get failed.

Change-Id: I5f59eb28baa96bef67434d54fb4ee73733d8c771