Add a metric decorator around cdm engine

[ Merge from http://go/wvgerrit/69105 ]

This adds a metric collecting decorator class around cdm engine. This
implementation uses a templated decorator. The decorator enables:

1) Wrapping the CDM Engine methods to capture timing and error
information.
2) Allows use of a mock CDM Engine for testing.

Test: Unit tests. GPlay manual testing and GTS tests.
BUG: http://b/64724336
Change-Id: I5e4a0f552974fab1939bc7ab02719a1f5849cf3f

libwvdrmengine/cdm/Android.mk

@@ -29,6 +29,7 @@ METRICS_SRC_DIR := metrics/src
 LOCAL_SRC_FILES := \
     $(CORE_SRC_DIR)/buffer_reader.cpp \
     $(CORE_SRC_DIR)/cdm_engine.cpp \
+    $(CORE_SRC_DIR)/cdm_engine_factory.cpp \
     $(CORE_SRC_DIR)/cdm_session.cpp \
     $(CORE_SRC_DIR)/cdm_session_map.cpp \
     $(CORE_SRC_DIR)/certificate_provisioning.cpp \