Improve OEMCrypto unit tests

This is a merge of http://go/wvgerrit/13701 and http://go/wvgerrit/13780.

I added a new set of engine properties for the mock oemcrypto.  This
set pretends to be level 1. This allows the widevine build bot to test
the dual security level path: a level 1 liboemcrypto.so and a fall
back to L3.

I also adjusted the failing test in oemcrypto_test.cpp.  A correct fix
requires us to rewrite some of the oemcrypto mock code so that it
returns real error codes instead of just 'false' on error.

Change-Id: I9cdbfc23c87ad2fb6068eac1394ce4c5b6a32dae

libwvdrmengine/oemcrypto/mock/src/oemcrypto_engine_mock.h

@@ -257,6 +257,7 @@ class CryptoEngine {
   bool closed_platform();
   bool supports_storage();
   bool is_anti_rollback_hw_present();
+  const char* security_level();
 
  private:
   SessionContext* current_session_;