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.

+ also use the correct user in InputMethodPreferences when checking for
  enforcing admin

Bug: 211063191
Test: manual, CtsVerifier/BYOD managed provisioning/Policy transparency
Change-Id: Ie8bf6de2d53c80b0f2f91e7e96479764834d55ef