framework/av: fixbug for mistake clear mainbuffer data,

root cause by not offset to fout

Test: 1. pair and connect with bluetooth ear phone;
      2. record video by camera ,play and pause again by
      loop test;
      3. Checking whether ear phone has pop sound.

Change-Id: Ief4117d634b9e7a9f27c669b8de7626b15df7636