Use _x() for passing strings as function parameters

[boonebgorges]

When passing strings as function parameters, as is done with bp_core_get_last_activity() here ​http://buddypress.trac.wordpress.org/browser/branches/1.6/bp-members/bp-members-template.php#L634, it's not at all clear from context what the arguments like %s should be. Here we shoud use _x() instead of () to provide the necessary context.

use _x() with a proper context when passing the string paramater to bp_core_get_last_activity()

(In [6615]) Use _x() to clarify string. Props jkudish. Fixes #4430.