DO NOT MERGE. Clarify FileObserver docs for null value.

Change-Id: I7dc0b0d26220d6eb82f956a9860cfb6aeccfd12f
Test: none
Bug: 30511606
(cherry picked from commit 902316d553)

core/java/android/os/FileObserver.java

@@ -16,6 +16,7 @@
 
 package android.os;
 
+import android.annotation.Nullable;
 import android.util.Log;
 
 import java.lang.ref.WeakReference;
@@ -204,7 +205,8 @@ public abstract class FileObserver {
      *
      * @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);
 }