Add DecryptCTR to OEMCrypto Mock

This is a software only implementation of the OEMCrypto library for
testing the rest of the DRM code.  It currently implements the
OEMCrypto_DecrtyptCTR function using a clear key.

I've included the license request code so the rest of the group can
play with it, but I have only tested part of it.

This patch also has some makefiles and an integration testing.  You
should be able to generate the shared library libclearkeydrmengine.so with
cd vendor/widevine/libclearkeydrmengine; mm
You can create some unit test and integration test programs from the
directories:
vendor/widevine/libwvdrmengine/oemcrypto/test
vendor/widevine/libclearkeydrmengine/test
vendor/widevine/libclearkeydrmengine/inttest
vendor/widevine/libclearkeydrmengine/crypto/test

This change also addresses some comments about comments in OEMCryptoDASH.h
which were made in https://googleplex-android-review.googlesource.com/257323

Change-Id: Id6899b9f8d2f09e09be2ea493baa83a6b929073b

libwvdrmengine/oemcrypto/mock/wvcrc32.h

@@ -0,0 +1,16 @@
+/*********************************************************************
+ * wvcrc32.h
+ *
+ * (c) Copyright 2011-2012 Google, Inc.
+ *
+ * Compte CRC32 Checksum. Needed for verification of WV Keybox.
+ *********************************************************************/
+
+#ifndef WV_CRC_32_H_
+#define WV_CRC_32_H_
+
+#include <stdint.h>
+
+uint32_t wvcrc32(uint8_t* p_begin, int i_count);
+
+#endif // WV_MOCK_OEMCRYPTO_H_