Use mac keys from usage entry if keys are not loaded

Merge from Widevine repo of http://go/wvgerrit/27461

In order to sign a license release message, the mac keys from the
usage entry should be used whenever keys have not been loaded.

This CL updates the reference code, the unit tests, and the level 3
oemcrypto.

b/38203566
Test: unit tests passing on bullhead.

Change-Id: Ic71fee4b4b7b45801548ab80fbbbf8f4ccab3e6e

libwvdrmengine/oemcrypto/test/oec_session_util.cpp

@@ -927,6 +927,9 @@ void Session::GenerateReport(const std::string& pst,
   EXPECT_GE(kHardwareSecureClock, pst_report().clock_security_level());
   EXPECT_EQ(pst.length(), pst_report().pst_length());
   EXPECT_EQ(0, memcmp(pst.c_str(), pst_report().pst(), pst.length()));
+  // Also, we the session to be able to sign the release message with the
+  // correct mac keys from the usage table entry.
+  ASSERT_NO_FATAL_FAILURE(VerifyClientSignature());
 }
 
 void Session::VerifyPST(const Test_PST_Report& expected) {