Clients don't get notifications of updates to the bluetooth.battery card property

While writing release notes, I noticed that the code that updates the bluetooth.battery card property doesn't send a subscription event to notify clients of the change. We could just slap a pa_subscription_post() call there and be done with it, but I would like to have a pa_card_set_property() that logs property changes and handles notifications instead.

We have functions pa_sink_input_set_property(), pa_sink_input_set_property_arbitrary() and pa_sink_input_update_proplist() which can be used as templates for implementing the same for cards.