Spotbugs: prevent SyncCredentials id field manipulation.

Author: greenrobot-team

objectbox-java/src/main/java/io/objectbox/sync/SyncCredentials.java

@@ -49,7 +49,7 @@ public enum CredentialsType {
 
         GOOGLE(2);
 
-        public long id;
+        public final long id;
 
         CredentialsType(long id) {
             this.id = id;