Added mutex protection for session_property_set_.

[ Merge of http://go/wvgerrit/154575 ]
[ Cherry-pick of http://ag/19498242 ]

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)
(cherry picked from commit 6109ec6d66)

libwvdrmengine/cdm/core/include/properties.h

@@ -142,6 +142,7 @@ class Properties {
   static void InitOnce();
 
   static std::mutex init_mutex_;
+  static std::mutex session_mutex_;
   static bool is_initialized_;
   static bool oem_crypto_use_secure_buffers_;
   static bool oem_crypto_use_fifo_;