DO NOT MERGE SoundPool: add lock for findSample access from SoundPoolThread

Sample decoding still occurs in SoundPoolThread
without holding the SoundPool lock.

Bug: 25781119
Change-Id: I11fde005aa9cf5438e0390a0d2dfe0ec1dd282e8

int SoundPool::beginThread(void* arg)
{
 SoundPool* p = (SoundPool*)arg;
 return p->run();
}

int SoundPool::beginThread(void* arg)
{
 SoundPool* p = (SoundPool*)arg;
 return p->run();
}
