Protect Session ID List With a Lock

(This is a merge of
https://widevine-internal-review.googlesource.com/#/c/11405
from the Widevine CDM Repo.)

AUPT is revealing a crash when destructing WVDrmPlugin due to
multi-threaded contention over the session map. As a fix, we are now
protecting access to the map via a mutex.

Bug: 17761616
Change-Id: Iddeca657effd3c7f3ff35ce334d7979291667cef

libwvdrmengine/mediadrm/include/WVDrmPlugin.h

@@ -16,6 +16,7 @@
 #include "utils/Errors.h"
 #include "utils/KeyedVector.h"
 #include "utils/List.h"
+#include "utils/Mutex.h"
 #include "utils/String8.h"
 #include "utils/Vector.h"
 #include "wv_cdm_event_listener.h"
@@ -26,6 +27,7 @@ namespace wvdrm {
 
 using android::KeyedVector;
 using android::List;
+using android::Mutex;
 using android::status_t;
 using android::String8;
 using android::Vector;
@@ -229,6 +231,7 @@ class WVDrmPlugin : public android::DrmPlugin,
 
   WvContentDecryptionModule* mCDM;
   WVGenericCryptoInterface* mCrypto;
+  Mutex mCryptoSessionsMutex;
   map<CdmSessionId, CryptoSession> mCryptoSessions;
 
   status_t mapAndNotifyOfCdmResponseType(const Vector<uint8_t>& sessionId,