Replace std::string with Privilege class

Replace std::string with Privilege class

New privileges (LICENSED/UNTRUSTED) needs to store
addtional information. Using pair of privilege and
license started to be confusing and requires a lot
of conversions. Privilege hides additional information
inside class that makes test more readable.

Change-Id: Ie5fd90e1a8077b7de1521ab1702905d7c7281eb2