Activity: refactor part of `bp_get_activity_latest_update` to use `bp_activity_get_permalink` to get a link to an activity item rather than manually constructing the link.

See #5926

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9063 cdf35c40-ae34-48e0-9cc9-0c9da1808c22