Correctly display popup dialogs from Emacsclient

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.