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_device_features.h

@@ -26,6 +26,7 @@ class DeviceFeatures {
   bool generic_crypto;     // Device supports generic crypto.
   bool cast_receiver;      // Device supports alternate rsa signature padding.
   bool usage_table;        // Device saves usage information.
+  bool supports_rsa_3072;  // Device supports 3072 bit RSA keys.
   uint32_t api_version;
   OEMCrypto_ProvisioningMethod provisioning_method;