Fixes to OakSessionConfigBuilder

* Create a private constructor that native code can use
* Make the loadLibrary call static so it's useable

Change-Id: I9b8586417f8c452565d3ded0345064b0dbf786b1

Author: jblebrun

java/src/main/java/com/google/oak/session/OakSessionConfigBuilder.java

@@ -25,7 +25,7 @@
  * as needed.
  */
 public class OakSessionConfigBuilder {
-  public void loadNativeLib() {
+  public static void loadNativeLib() {
     System.loadLibrary("oak_session_config_builder_jni");
   }
 
@@ -43,10 +43,14 @@ public enum HandshakeType {
     NOISE_NN,
   }
 
-  private final long nativePtr;
+  private long nativePtr;
 
   public OakSessionConfigBuilder(AttestationType attestationType, HandshakeType handshakeType) {
-    this.nativePtr = nativeCreateConfigBuilder(attestationType.ordinal(), handshakeType.ordinal());
+    this(nativeCreateConfigBuilder(attestationType.ordinal(), handshakeType.ordinal()));
+  }
+
+  private OakSessionConfigBuilder(long nativePtr) {
+    this.nativePtr = nativePtr;
   }
 
   public long getNativePtr() {