private void flushBytes() throws IOException {
        int position;
        if ((position = mBytes.position()) > 0) {
            mBytes.flip();
            mOutputStream.write(mBytes.array(), 0, position);
            mBytes.clear();
        }
    }
Allocate a FastXmlSerializer with the given internal output buffer size.  If the
size is zero or negative, then the default buffer size will be used.

@param bufferSize Size in bytes of the in-memory output buffer that the writer will use.
android/android_framework/base/src/main/java/github/tornaco/android/thanos/core/util/FastXmlSerializer.java