Test loading second call to LoadKeys fails

Merge from Widevine repo of http://go/wvgerrit/78144

This CL verifies that LoadKeys cannot be called twice a second time in
an OEMCrypto session.

Bug: 131359743
Test: unit tests on refernce code and taimen
Change-Id: I3da1c7639ed163799ee5996fd9f7f8b427c7ed30

libwvdrmengine/oemcrypto/ref/src/oemcrypto_session.cpp

@@ -636,6 +636,7 @@ OEMCryptoResult SessionContext::LoadKeys(
         break;
     }
   }
+  encryption_key_.clear();
   return OEMCrypto_SUCCESS;
 }