PermissionController: Implement getPermissionUsages API

This partially reverts commit fe4bf10f.

This restores the implementation of onGetPermissionUsages from the
aforementioned commit and updates it to work with the permission usage
forward-port in commit a48389aa.

Implementing this API is necessary to fix the permissions dashboard
entry point being empty in Settings.

Change-Id: If5e0cba6c87c1f1c7ef2550aeac405ccba7b3227