Activity copy is always sent FROM logged in user's email ID

Steps to replicate:

1. Create new Activity of type Meeting
2. Choose any contact other than logged in as 'Added by' (/source_contact_id)
3. Submit
4. A copy of the activity is sent to the target BUT the from email ID is the logged in user's email ID

This was purposely done under CRM-8926 at https://github.com/civicrm/civicrm-core/blob/master/CRM/Case/BAO/Case.php#L1324 so its not a regression

What expected is we should fetch the from email ID in following sequence:
1. Activity Added by Contact's email address
2. System Default From Address
3. Default Organization Contact email address
4. Logged in user