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

void SoundChannel::setVolume(float leftVolume, float rightVolume)
{
 Mutex::Autolock lock(&mLock);
    setVolume_l(leftVolume, rightVolume);
}

void SoundChannel::setVolume(float leftVolume, float rightVolume)
{
 Mutex::Autolock lock(&mLock);
    setVolume_l(leftVolume, rightVolume);
}
