fix: do not expose universe_domain yet (#1206)

Author: lsirac

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -580,7 +580,7 @@ public String getWorkforcePoolUserProject() {
   }
 
   @Nullable
-  public String getUniverseDomain() {
+  String getUniverseDomain() {
     return universeDomain;
   }