Binary compatibility validator: filter out empty facades *after* members are flattened from non-public part superclasses.

Use stable member sort order for members flattened from non-public superclasses, rewrite test to make order matter.

libraries/tools/binary-compatibility-validator/src/test/kotlin/cases/java/Facade.java

@@ -1,10 +1,10 @@
 package cases.java;
 
 class Part1 {
-    public static void publicMethod(String param) { }
+    public static void publicMethod(int param) { }
 
     public static class Part2 extends Part1 {
-        public static void publicMethod(int param) { }
+        public static void publicMethod(String param) { }
     }
 }