Report key status change

Also modified PolicyEngine::CanDecrypt to use the new Keys_status_.

Bug: 19771431

Merged from Widevine CDM repo:
https://widevine-internal-review.googlesource.com/#/c/13904/

Change-Id: If7fb7fa45ecf9ff4e65278cda00ad7230b57f50b

libwvdrmengine/mediadrm/src/WVDrmPlugin.cpp

@@ -954,6 +954,13 @@ void WVDrmPlugin::OnSessionExpiration(const CdmSessionId& cdmSessionId) {
   sendEvent(kDrmPluginEventKeyExpired, 0, &sessionId, NULL);
 }
 
+void WVDrmPlugin::OnSessionKeysChange(
+    const CdmSessionId& session_id,
+    const std::vector<CdmKeyInformation>& cdm_keys_info,
+    bool has_new_usable_key) {
+  // TODO(kqyang): Glue with DrmPlugin API when it is ready.
+}
+
 void WVDrmPlugin::OnExpirationUpdate(const CdmSessionId& cdmSessionId,
                                      int64_t new_expiry_time) {
   // TODO(kqyang): Glue with DrmPlugin API when it is ready. Note that