[0.6.189] keep the number of video buffer at attrs

[0.6.189] keep the number of video buffer at attrs

- Use attrs to keep the number of video buffers.
- It has to be considered when player export the
  'video decoded buffer' to client to avoid decoding error
  by buffer starvation.

Change-Id: I28d019b62df00266c3beccd63ad111f97563689d