Refactored user switch workflow to decouple UserInfo from TargetUser.

UserController calls SystemServiceManager on some stages of the user
switch lifecycle, and SystemServiceManager calls the respective
callbacks in the SystemService objects it manage.

Prior to Android 11, these callbacks would take a @UserIdInt userid,
but now they take a TargetUser (which in turns contains a Userinfo).
So, currently, UserController calls SystemServiceManager passing the
userid, and SystemServiceManager gets the UserInfo from
UserManagerService. That works fine most of the time, but there might
be some race conditions when a user is removed that can crash the
system when such user is stopped (because UserManagerService doesn't
have its UserInfo anymore).

This CL fixes this issue by refactoring the user switch workflow so:

- TargetUser doesn't hold a reference to the UserInfo anymore, but
  offer the methods needed by SystemService implementations.
- SystemServiceManager keeps track of the TargetUser objects used in
  the callbacks.

Test: manual verification

Fixes: 159831354

Change-Id: If68ab1b5e247c7389c8cac16432d8b8809507e86