Window order is not preserved when switching between user profiles

This was caused by the disposition change behavior of the focus controller,
which when we hide the windows of the first user one by one, it tried to
give focus to the next activateable window. This results in a change in
the most recent activated window before we switch to the new user. When we
return back to the original user, we find the order of windows had changed,
and this is the bug.

To avoid this bug, we need to make use of the MRU window tracker's tracked windows while hiding the old user's windows, in order to hide the MRU window last. Hence by the time it's hidden, there are no other windows activateable to give focus to.

BUG=424307
TEST=unit_tests --gtest_filter=MultiUserWindowManagerChromeOSTest.*

Review URL: https://codereview.chromium.org/1059473004

Cr-Commit-Position: refs/heads/master@{#327853}

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