Add some docs

    /**

     * Returns the system-wide unique id for this buffer

     *

     * This can be useful as a cache key for associating additional objects with

     * a given HardwareBuffer, such as associating an imported EGLImage with

     * the target HardwareBuffer when processing a stream of buffers from

     * ImageReader.

     *

     * This can also be useful for doing cross-process buffer caching. As sending

     * a HardwareBuffer over Binder is slower than sending a long, this can be

     * used as reliable cache key after an initial handshake that passes the

     * HardwareBuffers themselves to later be referred to using only the id.

     */

    public long getId() {

        checkClosed("id");