Simplify code relating to UI thread synchronization

Simplify code relating to UI thread synchronization

* java/org/gnu/emacs/EmacsContextMenu.java (display):

* java/org/gnu/emacs/EmacsDialog.java (display):

* java/org/gnu/emacs/EmacsService.java (getEmacsView)
(getLocationOnScreen, getClipboardManager)
(requestDirectoryAccess): Replace manual synchronization within
Runnable objects by usage of FutureTask.
(syncRunnable): Accept FutureTask<V> in place of Runnables, and
obtain and return results from calls to its get method.