Change from custom Lock to std::mutex.

[ Merge of http://go/wvgerrit/67884 ]

Now that we can use C++11, we should use the cross-platform std::mutex
type, not the custom pthread version.

Bug: 111850982
Test: WV unit/integration tests
Change-Id: If2fde2836826c5184609e6b1f3a6511206bd4594

libwvdrmengine/Android.mk

@@ -110,7 +110,6 @@ LOCAL_SRC_FILES := \
     $(UTIL_SRC_DIR)/clock.cpp \
     $(UTIL_SRC_DIR)/file_store.cpp \
     $(UTIL_SRC_DIR)/file_utils.cpp \
-    $(UTIL_SRC_DIR)/lock.cpp \
     $(UTIL_SRC_DIR)/log.cpp \
     $(SRC_DIR)/properties_android.cpp \
     $(SRC_DIR)/timer.cpp \