Change payment notification listener hook name

`SV_WC_Payment_Gateway_Hosted` automatically registers an action to listen for hosted transaction responses based on the value of `get_class()` -- [see it here](https://github.com/skyverge/wc-plugin-framework/blob/303895149120fb90f93093e4ff2a8e619914452b/woocommerce/payment-gateway/class-sv-wc-payment-gateway-hosted.php#L65-L68).

This has been fine in the past, but as we begin to move towards using namespaces in more plugins, this will become a problem as `get_class()` will result in the fully-qualified class name, including backslashes, therefore creating an action like `woocommerce_api_skyverge\woocommerce\plugin_name\gateway` which is pretty nasty-looking and won't work.

Instead, perhaps we should use the Gateway ID in the action rather than the class name.