CRM-18409 fix inability to view activity if source contact id is deleted.

CRM-18409 fix inability to view activity if source contact id is deleted.

This fix adds the ability to access activities through the api with the same permission checks as in the BAO if 'id' is passed in (and contact_id is not for 'supporting legacy stuff reasons').

Note that the api functionality is a good thing, but it is actually being added primarily for the purposes of being able to add a test fix for the change