Generate random data to fill the userid field

Since there is no resident key support on the relying party, generating
random data to fill the user.id field.

src/main/java/com/google/webauthn/gaedemo/objects/PublicKeyCredentialCreationOptions.java

@@ -58,7 +58,9 @@ public PublicKeyCredentialCreationOptions(String userName, String userId, String
     pubKeyCredParams = new ArrayList<PublicKeyCredentialParameters>();
     excludeCredentials = new ArrayList<PublicKeyCredentialDescriptor>();
     rp = new PublicKeyCredentialRpEntity(rpId, rpName, null);
-    user = new PublicKeyCredentialUserEntity(userName, userId.getBytes(StandardCharsets.UTF_8));
+    byte[] userIdBytes = new byte[32];
+    random.nextBytes(userIdBytes);
+    user = new PublicKeyCredentialUserEntity(userName, userIdBytes);
 
     challenge = new byte[CHALLENGE_LENGTH];
     random.nextBytes(challenge);