Added mutex protection for session_property_set_.

[ Cherry-pick of http://ag/19216679 ]
[ Merge of http://go/wvgerrit/155370 ]

There is a rare race condition experienced by some Android devices
where the a new client property set is being added while another is
being removed.  The C++ stl library does not provided thread
protection by default.

This CL adds a new mutex for the client property set map which prevents
multiple threads accessing the property sets concurrently.

Bug: 235238226
Test: GtsMediaTestCases on redfin
Change-Id: I32cf11bfb1332295ba1245071102ff0adc35259d
(cherry picked from commit aaa97a5d60)

libwvdrmengine/cdm/src/properties_android.cpp

@@ -52,7 +52,10 @@ void Properties::InitOnce() {
   allow_restore_of_offline_licenses_with_release_ =
       kAllowRestoreOfflineLicenseWithRelease;
   delay_oem_crypto_termination_ = kPropertyDelayOemCryptoTermination;
-  session_property_set_.reset(new CdmClientPropertySetMap());
+  {
+    std::unique_lock<std::mutex> lock(session_mutex_);
+    session_property_set_.reset(new CdmClientPropertySetMap());
+  }
 }
 
 bool Properties::GetCompanyName(std::string* company_name) {