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/test/oec_session_util.h

@@ -283,7 +283,8 @@ class LicenseRoundTrip
         update_mac_keys_(true),
         api_version_(kCurrentAPI),
         expect_request_has_correct_nonce_(true),
-        license_type_(OEMCrypto_ContentLicense) {}
+        license_type_(OEMCrypto_ContentLicense),
+        request_hash_() {}
   void CreateDefaultResponse() override;
   // Create a license with four keys. Each key is responsible for one of generic
   // encrypt (key 0), decrypt (key 1), sign (key 2) and verify (key 3). Each key