1. 首先将pfx证书放入assets文件夹中，例如放入assets/certificate.pfx。
2. 在应用程序中创建一个TrustStoreManager类，该类负责在设备级别上安装证书。

public class TrustStoreManager
{
    private static final String TRUSTSTORE_NAME = "certificate.pfx";
    private static final String TRUSTSTORE_PASSWORD = "password";
    
    public static void installCertificate(Context context)
    {
        try {
            KeyStore keystore = KeyStore.getInstance("PKCS12");
            InputStream is = context.getAssets().open(TRUSTSTORE_NAME);
            keystore.load(is, TRUSTSTORE_PASSWORD.toCharArray());
            
            TrustManagerFactory tmf = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
            tmf.init(keystore);

            SSLContext sslContext = SSLContext.getInstance("TLS");
            sslContext.init(null, tmf.getTrustManagers(), null);
            
            HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());
            is.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

3. 在应用程序的入口点中调用TrustStoreManager类的installCertificate方法以安装证书。

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TrustStoreManager.installCertificate(this);
    }
}

4. 接下来，在应用程序中使用以下代码来访问使用安装的证书的API。

URL url = new URL("https://example.com");
HttpsURLConnection urlConnection = (HttpsURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.setReadTimeout(10000);
urlConnection.setConnectTimeout(15000);
urlConnection.setDoInput(true);

InputStream is = urlConnection.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
String line = null;
while ((line = reader.readLine()) != null) {
    Log.d(TAG, line);
}

is.close();
urlConnection.disconnect();

此时，应用程序将使用设备级别上安装的证书来访问HTTPS API。