Add Mock OEMCrypto Stub

I added a Mock OEMCrypto implementation with stubs only.

I corrected the "extern C" location in OEMCrypto.h. (Thanks, Jerry!)

I added some Android.mk files that allows us to compile and
run a few simple unit tests.

Unit Test 1: The library compiles. (passes)
Unit Test 2: The test program links and runs. (passes)

Change-Id: Ib4c9554553de49645b5841b0bae889728cbfc8b1

libwvdrmengine/oemcrypto/test/Android.mk

@@ -0,0 +1,27 @@
+LOCAL_PATH:= $(call my-dir)
+
+include $(CLEAR_VARS)
+
+LOCAL_SRC_FILES:= \
+    TestOEMCrypto.cpp
+
+LOCAL_MODULE_TAGS := tests
+
+LOCAL_C_INCLUDES += \
+    bionic \
+    vendor/widevine/libwvdrmengine/oemcrypto/include \
+    vendor/widevine/libwvdrmengine/oemcrypto/mock
+
+
+LOCAL_SHARED_LIBRARIES := \
+    libdrmframework \
+    liblog \
+    libutils \
+    libz \
+    libcutils \
+    libdl \
+    libMockOEMCrypto            # This is used before a real OEMCrypto library exists.
+
+LOCAL_MODULE:=test-oemcrypto
+
+include $(BUILD_EXECUTABLE)