Correctly display popup dialogs from Emacsclient

* java/org/gnu/emacs/EmacsContextMenu.java (EmacsContextMenu):
Make subclasses final.
* java/org/gnu/emacs/EmacsDialog.java (display1): Check if an
instance of EmacsOpenActivity is open; if it is, try using it to
display the pop up dialog.
* java/org/gnu/emacs/EmacsDialogButtonLayout.java
(EmacsDialogButtonLayout): Make final.
* java/org/gnu/emacs/EmacsHolder.java (EmacsHolder<T>):
Likewise.
* java/org/gnu/emacs/EmacsOpenActivity.java (EmacsOpenActivity):
New field `currentActivity'.
(onCreate, onDestroy, onWindowFocusChanged, onPause): Set that
field as appropriate.

java/org/gnu/emacs/EmacsContextMenu.java

@@ -58,7 +58,7 @@ public final class EmacsContextMenu
   /* The last group ID used for a menu item.  */
   public int lastGroupId;
 
-  private static class Item implements MenuItem.OnMenuItemClickListener
+  private static final class Item implements MenuItem.OnMenuItemClickListener
   {
     public int itemID;
     public String itemName, tooltip;