Optimize R8 configuration

* java/org/gnu/emacs/EmacsWindowManager.java (WindowConsumer):
Mark as package-local.

* java/proguard.conf: List only symbols that must be preserved,
enabling R8 to optimize the remainder.

java/org/gnu/emacs/EmacsWindowManager.java

@@ -84,7 +84,7 @@ public final class EmacsWindowManager
     MANAGER = new EmacsWindowManager ();
   };
 
-  public interface WindowConsumer
+  interface WindowConsumer
   {
     public void attachWindow (EmacsWindow window);
     public EmacsWindow getAttachedWindow ();