Add mutex to CdmEngine for use of cert_provisioning_.

[ Merge of http://go/wvgerrit/167337 ]
[ PoC http://ag/21737796 ]

Bug: 258188673
Test: sts-tradefed run sts-dynamic-develop -m StsHostTestCases \
    -t android.security.sts.Bug_258188673
Test: GtsMediaTestCases
Change-Id: If71a0e7a81f376cf28688a590b6cb9dcea699545
Merged-In: If71a0e7a81f376cf28688a590b6cb9dcea699545

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -390,6 +390,8 @@ class CdmEngine {
   CdmSessionMap session_map_;
   CdmReleaseKeySetMap release_key_sets_;
   std::unique_ptr<CertificateProvisioning> cert_provisioning_;
+  // Lock must be acquired before using |cert_provisioning_|.
+  std::mutex cert_provisioning_mutex_;
   FileSystem* file_system_;
   Clock clock_;
   std::string spoid_;