LogoutUser relies on SYSTEM.supportsSwitchTo

logoutUser always switches to the system user. All that therefore
matters is that the device can, in fact, switch to the system user. This
is irrespective of whether it is HSUM or not, so we don't need any
special-casing; we can just request whether we can switch to user 0.
This is a general policy: we avoid checking modes and instead check
properties - in this case, whether SYSTEM_USER has the property allowing
us to switch to it.

Also fixes some UserControllerTest logoutUser tests that weren't working
properly since they weren't ever actually switching users.

Test: UserControllerTest
Bug: 411696141
Bug: 428046912
Flag: EXEMPT trivial equivalent code
Change-Id: I14bd7e92679c51f799737ca22faecf77fc9039b7