getUserLogoutability to return error for unsupported device

Since logoutUser() no longer supports HSUM devices that cannot switch to
headless system user, getUserLogoutability() should also reflect that in
the return value.

Also, getUserLogoutability() will not return error when target user is
not current, since logging out non-current user is already supported.

Bug: 380125011
Flag: android.multiuser.logout_user_api
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceMockedTest
Change-Id: I318df5a40d24788eca20bbc99533a5781a2382b3