[1/N] Replace global userId with per-function argument

To allow reusing DesktopTasksController functions that build WCTs for
desktop operations for cases where the intended user is not current
(such as during user-switching), this change removes the |userId| member
variable from inner functions are replaces it with a |userId| argument
to be passed by callers. The source of |userId|s become the
TaskInfo.userId when available or ShellController.currentUserId when we
do really mean to use the current user. ShellController is the source of
truth for Shell, so use that instead of manually tracking user id
changes.

A follow up change will do the same for usages of |taskRepository|.

This is a refactor to prepare for a future change where a transition
request with user change info requires restoring the active desk of an
incoming user (which would not be "current" yet to SystemUI/Shell) and
deactivating the activate desk of the outgoing user.

Flag: EXEMPT refactor
Bug: 430988310
Bug: 406255019
Test: atest WMShellUnitTest
Change-Id: Id4e34724b5728d6739ce7ff56ed93a09494c6ad3