IFace: update default implementation to use optional previewSurface

Bug: 194346408
Test: atest VtsHalBiometricsFaceTargetTest
Change-Id: I72d3d6d638a1662ebd1a53a7029ea3bf200efe48
diff --git a/biometrics/face/aidl/default/Session.h b/biometrics/face/aidl/default/Session.h
index 4d213e3..4152909 100644
--- a/biometrics/face/aidl/default/Session.h
+++ b/biometrics/face/aidl/default/Session.h
@@ -41,7 +41,7 @@
 
     ndk::ScopedAStatus enroll(const keymaster::HardwareAuthToken& hat,
                               EnrollmentType enrollmentType, const std::vector<Feature>& features,
-                              const NativeHandle& previewSurface,
+                              const std::optional<NativeHandle>& previewSurface,
                               std::shared_ptr<common::ICancellationSignal>* return_val) override;
 
     ndk::ScopedAStatus authenticate(