Update AudioFlinger comments

        virtual     status_t addEffectChain_l(const sp<EffectChain>& chain) = 0;

                    // remove an effect chain from the chain list (mEffectChains)

        virtual     size_t removeEffectChain_l(const sp<EffectChain>& chain) = 0;

                    // lock mall effect chains Mutexes. Must be called before releasing the

                    // lock all effect chains Mutexes. Must be called before releasing the

                    // ThreadBase mutex before processing the mixer and effects. This guarantees the

                    // integrity of the chains during the process.

                    // Also sets the parameter 'effectChains' to current value of mEffectChains.

                    void lockEffectChains_l(Vector<sp <EffectChain> >& effectChains);

                    // unlock effect chains after process

                    void unlockEffectChains(const Vector<sp<EffectChain> >& effectChains);

    public:

        enum mixer_state {

            MIXER_IDLE,

            MIXER_TRACKS_ENABLED,

            MIXER_TRACKS_READY

            MIXER_IDLE,             // no active tracks

            MIXER_TRACKS_ENABLED,   // at least one active track, but no track has any data ready

            MIXER_TRACKS_READY      // at least one active track, and at least one track has data

            // standby mode does not have an enum value

            // suspend by audio policy manager is orthogonal to mixer state

        };

        // playback track