Add mutex to CdmEngine for use of usage_session_.

[ Merge of http://go/wvgerrit/169430 ]

Bug: 263314813
Test: GtsMediaTestCases
Test: request_license_test
Change-Id: I0859865d26b14d492832001e55897f89853577f1

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -454,7 +454,10 @@ class CdmEngine {
   // API will release the handle to previously active secure stop license.
   std::unique_ptr<CdmSession> usage_session_;
   std::unique_ptr<UsagePropertySet> usage_property_set_;
-  int64_t last_usage_information_update_time_;
+  int64_t last_usage_information_update_time_ = 0;
+  // Should be acquired before changes to |usage_property_set_| or
+  // |usage_session_|.
+  std::mutex usage_session_mutex_;
 
   // Protect release_key_sets_ from non-thread-safe operations.
   std::mutex release_key_sets_lock_;