Keep OEMCrypto testbed singleton after terminate

Merge from Widevine repo of http://go/wvgerrit/72388

The unreleased oemcrypto test code has been modifed to keep a
singleton for the life of the process.  In order to do that, several
functions in the reference code have been made virtual, and some
initialization and termination has been moved from the constructor and
destructor to Initialize and Terminate.

Bug: http://b/120572363 Add Recoverable Errors (mod mock)
Test: unit tests
Change-Id: I300559195567a537c0700167514be0ea42363695

libwvdrmengine/oemcrypto/ref/src/oemcrypto_engine_ref.h

@@ -71,11 +71,11 @@ class CryptoEngine {
     return root_of_trust_.DeviceToken();
   }
 
-  virtual void Terminate() {}
+  virtual void Terminate();
 
   virtual SessionId OpenSession();
 
-  bool DestroySession(SessionId sid);
+  virtual bool DestroySession(SessionId sid);
 
   SessionContext* FindSession(SessionId sid);