BufferQueueConsumer: signal onFrameReleased on dropped frames

status_t BufferQueueConsumer::acquireBuffer(BufferItem* outBuffer,

        nsecs_t expectedPresent, uint64_t maxFrameNumber) {

    ATRACE_CALL();

    int numDroppedBuffers = 0;

    sp<IProducerListener> listener;

    {

        Mutex::Autolock lock(mCore->mMutex);

        // Check that the consumer doesn't currently have the maximum number of

                    // Front buffer is still in mSlots, so mark the slot as free

                    mSlots[front->mSlot].mBufferState = BufferSlot::FREE;

                    mCore->mFreeBuffers.push_back(front->mSlot);

                    listener = mCore->mConnectedProducerListener;

                    ++numDroppedBuffers;

                }

                mCore->mQueue.erase(front);

                front = mCore->mQueue.begin();

        ATRACE_INT(mCore->mConsumerName.string(), mCore->mQueue.size());

        mCore->validateConsistencyLocked();

    }

    if (listener != NULL) {

        for (int i = 0; i < numDroppedBuffers; ++i) {

            listener->onBufferReleased();

        }

    }

    return NO_ERROR;

}