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Fix race conditions in OobeConfiguration tests

Explicitly shut down LoginDisplayHost in OOBEConfigurationTests
It should remove flakiness upon shutdown.

Also Fix race condition in setting up demo mode (reason for previous revert).

Reland of https://chromium-review.googlesource.com/c/chromium/src/+/1934342

Bug: 997685
Change-Id: I0cb8f084484395a13bc3b29e8f1b7150cada8403
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942339
Commit-Queue: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720207}