Remove backward compatibility for check_is_admin

Remove backward compatibility for check_is_admin

This routine in policy.py used to have a backward compatibility
check to ensure proper behaviour even when the policy.json file
did not have a specific 'context_is_admin' policy.
However, this backward compatibility check does not work. It
appears indeed that it has been broken for several release cycles;
it is also possible that actually it never worked.
When the 'context_is_admin' policy is not in the policy.json file
the enforcer simply ends up evaluating whatever is the default
policy configured there.

Therefore this patch:
- Removes the backward compatibility check, since it does not work
- Fails, for safety, check_is_admin if 'context_is_admin' policy is
  not specified
- Fixeds check_is_advsvc in the same way (the backward compatibility
  check never made any sense for this function)
- Fixes unit tests adding appropriate tests for check_is_admin and
  check_is_advsvc

Change-Id: Ia47e5781d86a3f21b9d837c9ac70a62ac435d20b
Closes-Bug: #1445690