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_device_properties.cpp

@@ -39,4 +39,10 @@ bool CryptoEngine::is_anti_rollback_hw_present() {
   return false;
 }
 
+// Returns "L3" for a software only library.  L1 is for hardware protected
+// data paths.
+const char* CryptoEngine::security_level() {
+  return "L3";
+}
+
 }  // namespace wvoec_mock