Add performOperation stub.

Test: VtsAidlKeyMintTargetTest
Change-Id: I3689fd2993941343c6b9ba503fbe56153742fb3e
diff --git a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.h b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.h
index e79fe80..0aef81b 100644
--- a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.h
+++ b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.h
@@ -261,7 +261,7 @@
     ErrorCode UseRsaKey(const vector<uint8_t>& rsaKeyBlob);
     ErrorCode UseEcdsaKey(const vector<uint8_t>& ecdsaKeyBlob);
 
-  private:
+  protected:
     std::shared_ptr<IKeyMintDevice> keymint_;
     uint32_t os_version_;
     uint32_t os_patch_level_;