Add Unit Tests for Big RSA Keys

Merge from widevine of http://go/wvgerrit/22958/

This adds unit tests for OEMCrypto to check 3072 bit RSA keys.  It
also adds indicates that the mock supports them.

b/32776334

Change-Id: I68a3ff56969a860602e6c230558478dc3577c8f5

libwvdrmengine/oemcrypto/test/oec_session_util.cpp

@@ -191,7 +191,7 @@ void Session::GenerateDerivedKeysFromSessionKey() {
   GenerateNonce();
   vector<uint8_t> session_key;
   vector<uint8_t> enc_session_key;
-  PreparePublicKey();
+  if (public_rsa_ == NULL) PreparePublicKey();
   ASSERT_TRUE(GenerateRSASessionKey(&session_key, &enc_session_key));
   vector<uint8_t> mac_context;
   vector<uint8_t> enc_context;