Add unit test for preloaded license

Merge from Widevine repo of http://go/wvgerrit/96508

This adds a unit test for to verify that a preloaded license may be
loaded into OEMCrypto. A preloaded license is a license that does not
have a nonce, and for which there is no license request. This is used
in CAS and ATSC.

I also updated the test version string to
OEMCrypto unit tests for API 16.2. Tests last updated 2020-03-27

Bug: 144105097
Test: ran oemcrypto unit tests on taimen and with v16 modmock.
Change-Id: I6a4926917f36a084d15defa7b908d067612c4dcf

libwvdrmengine/oemcrypto/ref/src/oemcrypto_session.cpp

@@ -190,6 +190,7 @@ SessionContext::SessionContext(CryptoEngine* ce, SessionId sid,
       id_(sid),
       current_content_key_(nullptr),
       session_keys_(nullptr),
+      license_request_hash_(),
       rsa_key_(rsa_key),
       allowed_schemes_(kSign_RSASSA_PSS),
       decrypt_started_(false),