Transaction meet "CommandService Stateless invalid tx" warining message

[kenkan921]

Hi, I am new in both android app developing and hyperledger iroha. I am now designing an app with hyperledger iroha. When I was sending transaction request through the app to iroha server, the server-side showed below message:
[2018-11-30 05:46:16.131450474][th:118][warning] CommandService Stateless invalid tx: Signature: [[Wrong signature [073920b66487c3e6196db291cba7815236c24f9629a82a785bf9c852a79befbc13382e2939797b4683d68e88cf0adb2c35908b02ef41c5da67cfac02649b0505;313a07e6384776ed95447710d15e59148473ccfc052a681317a72a69f2a49910] ]] Transaction: [[Transaction should contain at least one command Wrongly formed creator_account_id, passed value: ''. Field should match regex '[a-z_0-9]{1,32}\@([a-zA-Z]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)*[a-zA-Z]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?' bad timestamp: too old, timestamp: 0, now: 1543556776120 Quorum should be within range (0, 128] ]] , hash: 708713afdb215a7fdd1b3246ec662a3d992648dc262ca1d5360be94b6d75313b

The error returned from my app is "Error: java.lang.RuntimeException: Transaction failed " and without any other information.

The code transaction part in my app is simply copied from iroha-android-sample, i.e.

protected  Completable TransferAsset(String[] data){
    return Completable.create(emitter -> {
       long currentTime = System.currentTimeMillis();
       Keypair userKeys = crypto.convertFromExisting(PUB_KEY, PRIV_KEY);
       String username = "admin";

       UnsignedTx sendAssetTx = txBuilder.creatorAccountId(username + "@" + DOMAIN_ID)
               .createdTime(BigInteger.valueOf(currentTime))
               .transferAsset(username + "@" + DOMAIN_ID,
                       data[0] + "@" + DOMAIN_ID, "hkd#test", "initail", data[1])
               .build();

       protoTxHelper = new ModelProtoTransaction(sendAssetTx);
       ByteVector txblob = protoTxHelper.signAndAddSignature(userKeys).finish().blob();
       byte[] bsq = toByteArray(txblob);
       TransactionOuterClass.Transaction protoTx = null;

       try{
           protoTx = TransactionOuterClass.Transaction.parseFrom(bsq);
       }catch (InvalidProtocolBufferException e){
           emitter.onError(e);
       }

        CommandServiceGrpc.CommandServiceBlockingStub stub = CommandServiceGrpc.newBlockingStub(channel)
                .withDeadlineAfter(CONNECTION_TIMEOUT_SECONDS, TimeUnit.SECONDS);

       stub.torii(protoTx);

       if (!isTransactionSuccessful(stub, sendAssetTx)){
           emitter.onError(new RuntimeException("Transaction failed"));
       } else {
           emitter.onComplete();
       }

    });
}

I found same issue was reported by truongnmt but no one answered. Can anyone tell what is this issue and teach me how to solve it?