Avoid multiple loads of Device RSA private key

Molly reports OEMCrypto errors when Device RSA private key is
loaded a second time in the same session. This occurs in privacy
mode when a service certificate is request and later a key request is
generated.

bug: 10815492

Merge of https://widevine-internal-review.googlesource.com/#/c/7823/
from Widevine cdm repo

Change-Id: I98999fb0e77597109b68c379eaaa4838d3f6dde4

libwvdrmengine/cdm/core/include/cdm_session.h

@@ -120,6 +120,7 @@ class CdmSession {
 
   // Used for certificate based licensing
   std::string wrapped_key_;
+  bool is_certificate_loaded_;
 
   std::set<WvCdmEventListener*> listeners_;