Add back in deprecated methods in ServiceAccountJwtAccessCredentials (#238)

* Add back in deprecated methods in ServiceAccountJwtAccessCredentials

* typo fix

* change constructor to private

* change constructor to private

Author: kolea2

oauth2_http/java/com/google/auth/oauth2/ServiceAccountJwtAccessCredentials.java

@@ -96,6 +96,22 @@ public class ServiceAccountJwtAccessCredentials extends Credentials
   @VisibleForTesting
   transient Clock clock = Clock.SYSTEM;
 
+  /**
+   * Constructor with minimum identifying information.
+   *
+   * @param clientId Client ID of the service account from the console. May be null.
+   * @param clientEmail Client email address of the service account from the console.
+   * @param privateKey RSA private key object for the service account.
+   * @param privateKeyId Private key identifier for the service account. May be null.
+   * @deprecated Use {@link #newBuilder()} instead. This constructor will either be deleted or made
+   *             private in a later version.
+   */
+  @Deprecated
+  public ServiceAccountJwtAccessCredentials(
+      String clientId, String clientEmail, PrivateKey privateKey, String privateKeyId) {
+    this(clientId, clientEmail, privateKey, privateKeyId, null);
+  }
+
   /**
    * Constructor with full information.
    *