IFace: update default implementation

Bug: 170651283
Test: atest VtsHalBiometricsFaceTargetTest
Change-Id: I6f8e6ec12e597034264e2b1383bc7325b0f697b7
diff --git a/biometrics/face/aidl/default/Session.h b/biometrics/face/aidl/default/Session.h
index caf81f8..4d213e3 100644
--- a/biometrics/face/aidl/default/Session.h
+++ b/biometrics/face/aidl/default/Session.h
@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include <random>
+
 #include <aidl/android/hardware/biometrics/face/BnSession.h>
 #include <aidl/android/hardware/biometrics/face/ISessionCallback.h>
 
@@ -68,6 +70,7 @@
 
   private:
     std::shared_ptr<ISessionCallback> cb_;
+    std::mt19937 mRandom;
 };
 
 }  // namespace aidl::android::hardware::biometrics::face