put gameover alert on backstack

This seems to do what I want: cause it to be dismissed whenever the
board fragment behind it (that launched it) goes away, e.g. because the
remote deleted first and local got a deleted-elsewhere-so-delete-here?
alert.

xwords4/android/app/src/main/java/org/eehouse/android/xw4/GameOverAlert.java

@@ -128,6 +128,9 @@ public class GameOverAlert extends XWDialogFragment
     @Override
     protected String getFragTag() { return TAG; }
 
+    @Override
+    public boolean belongsOnBackStack() { return true; }
+
     @Override
     public void onClick( DialogInterface dialog, int which )
     {