Error https request android <20

Question

I'm trying to develop an app to communicate with amazon alexa using OKhttp3. Everything was working fine while I was trying the app on an android API 26. But once I started using an android API 19 every request to https://avs-alexa-na.amazon.com/ started failing.

Before working for API 19 this was how I used this function to open the down channel.

    final String url = "https://avs-alexa-na.amazon.com/";
    final String version = "v20160207";
    final Request request = new Request.Builder()
            .get()
            .header("Authorization", "Bearer " + myApplication.getAccessToken())
            .url(url + version + "/directives")
            .build();
    try {
        final OkHttpClient client = new OkHttpClient();
        Response response = client.newCall(request).execute();
        Log.i("Down channel", "opened");
    } catch (Exception ex) {
        Log.e("Down channel", "error Sending request " + ex.getLocalizedMessage());
    }

But now this same function returns javax.net.ssl.SSLProtocolException: SSL handshake aborted: ssl=0xb969de80: Failure in SSL library, usually a protocol error error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure (external/openssl/ssl/s23_clnt.c:741 0x8d954990:0x00000000)

So after following some posts on line I learned that android 19 doesn't install by default TLSv1.2 so I added a function to install it and started having this error :unexpected end of stream on Connection{avs-alexa-na.amazon.com:443, proxy=DIRECT@ hostAddress=avs-alexa-na.amazon.com/54.239.39.74:443 cipherSuite=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 protocol=http/1.1}

I started exploring other ways working with certificates (my certificate, and the one from amazon gotten by openssl s_client -connect avs-alexa-eu.amazon.com:443:

SSLContext sslContext;
    TrustManager[] trustManagers;
    try {
        final KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
        keyStore.load(null, null);
        final InputStream certInputStream = activity.getResources().openRawResource(R.raw.cert);
        InputStream avscertInputStream = activity.getResources().openRawResource(R.raw.amazon_avs_cert);
        final CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
        final Certificate cert = certificateFactory.generateCertificate(certInputStream);
        final Certificate avscert = certificateFactory.generateCertificate(avscertInputStream);
        keyStore.setCertificateEntry("amazon_avs", avscert);
        keyStore.setCertificateEntry("cert", cert);
        final TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
        trustManagerFactory.init(keyStore);
        trustManagers = trustManagerFactory.getTrustManagers();
        sslContext = SSLContext.getInstance("TLSv1.2");
        sslContext.init(null, trustManagers, null);
        return new OkHttpClient.Builder()
                .sslSocketFactory(sslContext.getSocketFactory(), (X509TrustManager) trustManagers[0])
                .build();
    } catch (Exception e) {
        e.printStackTrace();
        return new OkHttpClient();
    }

But it led to a java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

I'm lost, I don't know what I did wrong or what I did not do. Thanks for you help.