Test Simultaneous decrypt and remove NULL pointer comparison

[ Merge of http://go/wvgerrit/16544, http://go/wvgerrit/16639 ]

* This fixes the oemcrypto unit tests to build with the ce cdm.

The unit tests do not build when it is detected that a long (NULL)
is compared to a pointer.

* Remove NULL pointer comparison

On some platforms ASSERT_NE(NULL, ptr) does not work.  This CL
replaces it with ASSERT_TRUE(NULL != ptr).

* Test Simultaneous Decrypt

With the increasing number of devices that support multiple screens or
windows, it is desireable to verify that OEMCrypto can have several
sessions open and actively decrypting at the same time.

Calls to OEMCrypto are still serialized -- this is not a threading
test -- but we still have multiple sessions open and decrypt from each
of them.

* Remove unused variable in initialization_data

Change-Id: I1a4be38fb30a14f610544416db653a81342f16b3

libwvdrmengine/cdm/core/src/initialization_data.cpp

@@ -357,7 +357,6 @@ bool InitializationData::ConstructWidevineInitData(
                           json_init_data.size());
 
   // Parse the Json string using jsmn
-  int result = 0;
   jsmn_parser parser;
   jsmntok_t tokens[kDefaultNumJsonTokens];
   jsmn_init(&parser);