Use filtered content when storing latest update in usermeta

Previously, the content was run through the
bp_activity_latest_update_content filter, but was then unused.

Fixes #4892

Props saurabhshukla

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7115 cdf35c40-ae34-48e0-9cc9-0c9da1808c22