Update HandlerThread's javadoc

import android.annotation.Nullable;

/**

 * Handy class for starting a new thread that has a looper. The looper can then be 

 * used to create handler classes. Note that start() must still be called.

 * A {@link Thread} that has a {@link Looper}.

 * The {@link Looper} can then be used to create {@link Handler}s.

 * <p>

 * Note that just like with a regular {@link Thread}, {@link #start()} must still be called.

 */

public class HandlerThread extends Thread {

    int mPriority;