Use `bp_update_user_meta()` in `bp_update_user_last_activity()` to ensure `bp_get_user_meta_key` filter is ran.

Fixes bug causing `last_activity` meta to not be prefixed, and includes testcase.

Fixes #5988.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9123 cdf35c40-ae34-48e0-9cc9-0c9da1808c22