More SoundPool cleanup. Remove mSoundPoolRef and unused notify callback.

    SoundPool() {} // no default constructor

    bool startThreads();

    void doLoad(sp<Sample>& sample);

    inline void notify(const SoundPoolEvent* event) {

        android_soundpool_SoundPool_notify(mSoundPoolRef, event);

    }

    sp<Sample> findSample(int sampleID) { return mSamples.valueFor(sampleID); }

    SoundChannel* findChannel (int channelID);

    SoundChannel* findNextChannel (int channelID);

    int run();

    void quit();

    jobject                 mSoundPoolRef;

    Mutex                   mLock;

    Mutex                   mRestartLock;

    Condition               mCondition;