Frameworks/av: Fix for mediaserver crash in AudioFlinger

- UI freeze is observed while playing LPA clips with sound effects.
- While moving to next clip, effect process is still applying
  effects on last buffers. Meanwhile de-allocating the local buffers
  in DirectAudioTrack destructor on which effects are currently
  applying. Due to this crash is observed in effect process.
- De-allocate buffers once the effects thread is killed.

Change-Id: I67431f298b8da114db60de5cc27376d9ddce7da5
CRs-Fixed: 416093