Deprecate NotificationFilterPreference#isActive

The concept of active notification filter preferences does not make sense anymore, the javadoc says:

A filter preference can either be active or passive. It the preference is active, then it should force the retrieval of notifications when used in conjunction with a {@link NotificationFilter}.

On the other hand, a passive (non-active) notification filter should not automatically trigger the retrieval of notifications.

This was probably valid back then when notifications were using a post-filtering mechanism to be retrieved, but now that we're using prefiltering by default it doesn't make sense anymore.