Convert 12 words mnemonic to Private Key in Ethereum

Question

I'm making an Android application where users can login using their Ethereum account and make transactions. So far, the user can login with his private key. I want to add the option of using the 12 mnemonic words to login as well.

I am able to generate the seed (mnemonic + passphrase) using MnemonicUtils: https://github.com/web3j/web3j/blob/master/crypto/src/main/java/org/web3j/crypto/MnemonicUtils.java. But how can I generate the private key of the account from that seed? I have already tried using the code below, but it generates a different private key everytime for the exact same mnemonic.

String mnemonic="" //I put here my own mnemonic
ECKeyPair exKey= null;
        try {

               exKey = Keys.createEcKeyPair();
                } catch (InvalidAlgorithmParameterException e) {
                e.printStackTrace();
                } catch (NoSuchAlgorithmException e) {
                e.printStackTrace();
                } catch (NoSuchProviderException e) {
                e.printStackTrace();
                }

                WalletFile wallet=null;
                try {
                wallet = Wallet.createLight(mnemonic,exKey);
                } catch (CipherException e) {
                e.printStackTrace();
                }

                Credentials credentials = null;
                try {
                credentials = Credentials.create(Wallet.decrypt(mnemonic,wallet));
                } catch (CipherException e) {
                e.printStackTrace();
                }
String privateKeyGenerated = credentials.getEcKeyPair().getPrivateKey().toString(16);
                Log.d("PK", privateKeyGenerated);