Rename logout methods in UserRepository for extra clarity

This is a preliminary work for fixing 404183874

There are two actions in Android that are called "Logout".

The older one is the process of returning from secondary user
into a primary one on devices managed by DO, where user switch
from primary user to secondary was initiated via
DevicePolicyManager, and secondary user might be restricted
from simply switching to primary user via user restrictions.

Recent one is being added to support desktop-like user interaction
where instead of directly switching between different users
they have to end session for one user, return to neutral state
(login screen) that is ran under system user, and only then
start new user session. This logic is contained in UserManager.

While both of these actions are called "logout", they have quite
different underlying logic. As this logic might evolve with time
(especially for UserManager path) it better to add a bit more clarity
in method names, so it does not lead to confusion like
ag/32246147 that fixed b/401000285 but lead to b/404183874.

Test: m sysui
Bug: 404183874
Flag: EXEMPT Pure refactoring (rename)
Change-Id: I0b1c399e2fadbe66ff670277f7f73e519283f448