Make distinction between null and empty IM list.

* null means no policy is enfrced, any IM is allowed.
* empty means no IMs is allowed except system IMs.

Bug: 211063191
Test: manual, CtsVerifier/BYOD managed provisioning/Policy transparency
Change-Id: I35665a547c1e99129c70c834083b3183ba00208d
1 file changed