Use separate "logout eligibility" logic for managed and desktop logouts

Ability to logout residing in DevicePolicyManager relies on feature
being enabled by admin and having a remembered user system can switch to
upon logout.

Latter one is not relevant for logout logic implemented by UserManager,
that provides separate UserManager.getUserLogoutability check.

Also reordered a bit a logic inside Flows to have quick synchronous
checks run before diving into *Manager calls.

Test: atest UserRepositoryImplTest
Fixes: 404183874
Flag: EXEMPT bugfix
Change-Id: I7860df352613b4e723b9fe40d7846d3570d2496b