[ads] Enable BraveSearchAdStudy by default

[tmancey]

Resolves https://github.com/brave/brave-browser/issues/36604

Submitter Checklist:

• [x] I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• [x] There is a ticket for my issue
• [x] Used Github auto-closing keywords in the PR description above
• [x] Wrote a good PR/commit description
• [x] Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• [x] Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• [x] Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• [x] Ran git rebase master (if needed)

Reviewer Checklist:

• [ ] A security review is not needed, or a link to one is included in the PR description
• [ ] New files have MPL-2.0 license header
• [ ] Adequate test coverage exists to prevent regressions
• [ ] Major classes, functions and non-trivial code blocks are well-commented
• [ ] Changes in component dependencies are properly reflected in gn
• [ ] Code follows the style guide
• [ ] Test plan is specified in PR before merging

After-merge Checklist:

• [ ] The associated issue milestone is set to the smallest version that the changes has landed on
• [ ] All relevant documentation has been updated, for instance:
  • [ ] https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • [ ] https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • [ ] https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • [ ] https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • [ ] https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • [ ] https://github.com/brave/brave-browser/wiki/QA-Guide
  • [ ] https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[github-actions[bot]]

[puLL-Merge] - brave/brave-core@22838

Description

This PR makes several changes related to ads features and time utilities. The main motivation appears to be enabling some ads features by default and cleaning up some time utility functions.

Changes

Changes

browser/brave_ads/ad_units/search_result_ad/search_result_ad_browsertest.cc

• Changes the feature list initialization to disable kShouldAlwaysRunBraveAdsServiceFeature and kShouldAlwaysTriggerBraveSearchResultAdEventsFeature in SearchResultAdTest

components/brave_ads/core/internal/ad_units/search_result_ad/search_result_ad_test.cc

• Adds scoped feature lists to disable kShouldAlwaysTriggerBraveSearchResultAdEventsFeature in some test cases

components/brave_ads/core/internal/ads_feature.cc

• Enables kShouldAlwaysRunBraveAdsServiceFeature, kShouldSupportSearchResultAdsFeature, and kShouldAlwaysTriggerBraveSearchResultAdEventsFeature features by default

components/brave_ads/core/internal/ads_feature_unittest.cc

• Adds scoped feature lists to disable features in some negative test cases

components/brave_ads/core/internal/common/time/time_util.cc, components/brave_ads/core/internal/common/time/time_util.h, components/brave_ads/core/internal/common/time/time_util_unittest.cc

• Removes several "TODO" functions related to working around base::Time::FromLocalExploded issues on Linux
• Removes the g_from_local_exploded_failed_for_testing test hook
• Removes all the Linux-specific unit tests using ScopedLibcTZ

Security Hotspots

None. The changes do not appear to introduce any new security vulnerabilities. Enabling some features by default seems intentional.