CRM-17645 breaks Activity screen when CiviCase is enabled

This is related to CRM-18309 (and thus CRM-17645), but is much more serious, so I'm giving it its own issue.

This can't be replicated on the demo server because it requires access to CMS permissions, but I replicated it on a clean install of dmaster using civibuildkit.

Steps to replicate:

• Remove all CiviCase permissions from a role (I used Administrator).
• Leave CiviCase enabled.
• Log in with a user with that role who is NOT Drupal user 1 (user 1 has all permissions regardless of role)
• Visit the Activities tab of any contact with at least one activity.

You'll see the error:
DataTables warning: table id=DataTables_Table_0 - Invalid JSON response. For more information about this error, please see http://datatables.net/tn/1

The root cause is the same as CRM-18309: The AJAX for DataTables is returning the error: "API permission check failed for Case/getcount call; insufficient permission: require Array". However, the fix I posted there does NOT work here. This confirms my belief that this needs fixing at a lower level.

Workarounds:

• Grant the CiviCase permissions to all roles.
• Disable CiviCase.

Aggravating factors:

• "CiviCase enabled without giving everyone all permissions" is a very common configuration. Especially since new CiviCRM downloaders seem to enable all components.
• This is a recent regression - I tested and confirmed that this problem doesn't exist in 4.7.4.
• Activity tab is more prominent than Option Values, which CRM-18309 deals with.