frameworks/base: Fix for seek issue while LPA Playback

- During LPA playback if we drag the progress bar to seek
  several times, it is observed that audio will play the
  sound of previous position for 2-3 sec.
- When seek command is issued decoder thread is not blocked, which
  is trying to send data available at old position. Due to this we hear
  data available at old position for few seconds.
- Block the decoder thread till all the buffers are flushed

Change-Id: If9e8db0cb0faf6ed0aa27748b5ef6f0ab392ec72
CRs-fixed: 358928