8261880: Change nested classes in java.base to static nested classes where possible

Reviewed-by: redestad

src/java.base/share/classes/java/security/ProtectionDomain.java

@@ -566,6 +566,6 @@ public class ProtectionDomain {
     /**
      * Used for storing ProtectionDomains as keys in a Map.
      */
-    final class Key {}
+    static final class Key {}
 
 }