Resets the previous time zone after AutofillAblationStudyTest.

icu::TimeZone::adoptDefault() is documented as restoring the default
time zone if nullptr is passed, but the actual implementation returns
without doing anything at all. This causes subsequent tests (for
example, DeviceEventLogTest.TestTimeFormat) to fail, because ICU
remains set to a non-local time zone.

This CL explicitly saves and restores the previous time zone after
AutofillAblationStudyTest is complete.

Bug: 342183883
Change-Id: I233780163bf166a73d8d9b7ebdc47a1733eb0f05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608197


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312630}