DO NOT MERGE. Clarify FileObserver docs for null value.

package android.os;

import android.annotation.Nullable;

import android.util.Log;

import java.lang.ref.WeakReference;

     *

     * @param event The type of event which happened

     * @param path The path, relative to the main monitored file or directory,

     *     of the file or directory which triggered the event

     *     of the file or directory which triggered the event.  This value can

     *     be {@code null} for certain events, such as {@link #MOVE_SELF}.

     */

    public abstract void onEvent(int event, String path);

    public abstract void onEvent(int event, @Nullable String path);

}