Stress test for AudioTrack shared memory bug

Changes in patch set 2:
 - add call to ProcessState::self()->startThreadPool()
 - remove dead code AudioSystem::get_audio_flinger
 - use simpler MemoryDealer isntead of MemoryHeapBase
 - AudioTrack now must be sp<> instead of raw pointer
 - use new constants from audio.h
 - change LOGx macros to ALOGx
 - fix file permission on Android.mk

Bug: 2801375
Change-Id: I899b08e0aa91deca328523f66f71e667011b4522