Restructed reference root of trust (3/3 OEM Cert)

[ Merge of http://go/wvgerrit/116944 ]

This change is the last part of a three part change for restructing
the root of trust used by the reference implementation.

OEM Certificates are now managed by the root of trust of the crypto
engine.  Previously, OEM certs where handled separately on a session
by session basis.

Bug: 135283522
Test: oemcrypto_unittests ce_cdm_tests
Change-Id: I6cf1fa3fade28baad85b5fce57a8eab6f2ed17c1

libwvdrmengine/oemcrypto/ref/src/oemcrypto_session.h

@@ -75,6 +75,9 @@ class SessionContext {
       const std::vector<uint8_t>& enc_session_key,
       const std::vector<uint8_t>& mac_context,
       const std::vector<uint8_t>& enc_context);
+
+  virtual OEMCryptoResult LoadOemPrivateKey();
+
   virtual OEMCryptoResult PrepAndSignLicenseRequest(uint8_t* message,
                                                     size_t message_length,
                                                     size_t* core_message_length,