cleanup: remove unique_ptrs from AutoReset variables in kiosk code

As far as I can tell, these unique_ptrs were probably added because some
of the callsites have to conditionally create these AutoReset variables,
and AutoReset lacks a no-args constructor.

I changed to the code to own a std::optional in these rare cases, and
to directly instantiate the AutoReset variables at construction time in
the other cases.

Bug: N/A
Test: browser_tests
Change-Id: Ibf2d5f123329f4a0ab42d56e4a58ccc1d4002b82
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5339610
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@google.com>
Commit-Queue: Jeroen Dhollander <jeroendh@google.com>
Cr-Commit-Position: refs/heads/main@{#1271423}