stagefright: add nonblocking MediaBufferGroup.acquire_buffer method

    void add_buffer(MediaBuffer *buffer);

    // Blocks until a buffer is available and returns it to the caller,

    // the returned buffer will have a reference count of 1.

    status_t acquire_buffer(MediaBuffer **buffer);

    // If nonBlocking is false, it blocks until a buffer is available and

    // passes it to the caller in *buffer, while returning OK.

    // The returned buffer will have a reference count of 1.

    // If nonBlocking is true and a buffer is not immediately available,

    // buffer is set to NULL and it returns WOULD_BLOCK.

    status_t acquire_buffer(MediaBuffer **buffer, bool nonBlocking = false);

protected:

    virtual void signalBufferReturned(MediaBuffer *buffer);

    mLastBuffer = buffer;

}

status_t MediaBufferGroup::acquire_buffer(MediaBuffer **out) {

status_t MediaBufferGroup::acquire_buffer(

        MediaBuffer **out, bool nonBlocking) {

    Mutex::Autolock autoLock(mLock);

    for (;;) {

            }

        }

        if (nonBlocking) {

            *out = NULL;

            return WOULD_BLOCK;

        }

        // All buffers are in use. Block until one of them is returned to us.

        mCondition.wait(mLock);

    }