activity filter user preference: flush cache before setting

when a user select either the include/exclude activity type filter on the contact's activity tab, we remember the setting and set it as a default on other records visited.

however, the behavior is acting erratically because user preferences may be stored in the global cache (e.g. memcache), and not reliably updated when the new filter value is set. we should flush the cache before setting the value to ensure we always retrieve the most recent setting.