Simplify initialization of Settings

There was a weird circular class-loading dependency between Settings and
Settings.Builder that could result in ClassLoader deadlock if multiple
threads trigger class-loading of Settings at the same time.

Signed-off-by: Michael Froh <froh@amazon.com>

server/src/main/java/org/opensearch/common/settings/Settings.java

@@ -101,7 +101,7 @@
 @PublicApi(since = "1.0.0")
 public final class Settings implements ToXContentFragment {
 
-    public static final Settings EMPTY = new Builder().build();
+    public static final Settings EMPTY = new Settings(Collections.emptyMap(), null);
 
     /** The raw settings from the full key to raw string value. */
     private final Map<String, Object> settings;
@@ -757,7 +757,7 @@ public Set<String> keySet() {
     @PublicApi(since = "1.0.0")
     public static class Builder {
 
-        public static final Settings EMPTY_SETTINGS = new Builder().build();
+        public static final Settings EMPTY_SETTINGS = Settings.EMPTY;
 
         // we use a sorted map for consistent serialization when using getAsMap()
         private final Map<String, Object> map = new TreeMap<>();