OEMCrypto Mock should not downgrade SRM

Merge from Widevine repo of http://go/wvgerrit/26560

With this CL, the modifiable oemcrypto mock will only update the SRM
version number if the new number is greater than or equal to the
current one.

b/37744957

Change-Id: I10721146ca0ce24b1087657e18569a434e5d53b8

libwvdrmengine/oemcrypto/mock/src/oemcrypto_engine_device_properties_mod.cpp

@@ -374,6 +374,11 @@ class AndroidModifiableCryptoEngine : public CryptoEngine {
     }
     int new_version = GetOption("srm_load_version", -1);
     if (new_version >= 0) {
+      if (new_version < srm_version_) {
+        LOGE("New SRM version is lower than existing SRM version: %d < %d",
+             new_version, srm_version_);
+        return OEMCrypto_ERROR_INVALID_CONTEXT;
+      }
       srm_version_ = new_version;
       LOGI("OEMCrypto mock told to change SRM version to %d.", srm_version_);
       srm_loaded_ = true;