Dominik/ntp search bar experiment

[ayoy]

Task/Issue URL: https://app.asana.com/0/72649045549333/1208262570807667/f

Description:
This change adds pixels for tracking NTP Search Box experiment.

Steps to test this PR:

1. Update DefaultNewTabPageSearchBoxExperimentCohortDecider.cohort to return .control
2. Run the app and enable internal user state, then close and reopen window.
3. Verify that you're not seeing the search bar and the logs say "User assigned to cohort control"
4. Update the code to return .experiment cohort.
5. Clean app data and repeat step 2.
6. Verify that you're seeing the search bar and the logs say "User assigned to cohort ntp_search_box"

Verifying experiment pixel:

1. Filter Xcode console logs by m_mac_initial-search.
2. Search for something from NTP search bar, verify that m_mac_initial-search-day-1 is sent with a count = 1 and from = ntp_search_bar
3. Search for something from NTP address bar, verify that m_mac_initial-search-day-1 is sent with a count = 2 and from = ntp_address_bar
4. Search for something from another non-NTP tab, verify that m_mac_initial-search-day-1 is sent with a count = 3 and from = address_bar
5. Repeat searches until you reach count = 10
6. Search again and verify that no pixel is sent.
7. Set the date in the operating system to +1 day.
8. Do a search, verify that that m_mac_initial-search-day-2 is sent with a count = 1 and appropriate from parameter.
9. Before continuing with the last step, complete Verifying autocomplete pixel parameters now.
10. Set the date to +7 days.
11. Do a search, verify that no pixel is sent, and autocomplete pixels don't have extra parameters.

Verifying autocomplete pixel parameters:

1. Type a phrase in the address bar, NTP search box or NTP address bar.
2. Select an autocomplete suggestion (phrase, website, bookmark, favorite or history item)
3. Verify that an appropriate autocomplete pixel is sent (m_mac_autocomplete_*) with the correct from parameter, and the experiment cohort.

Definition of Done:

• Does this PR satisfy our Definition of Done?

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Internal references:

Pull Request Review Checklist
Software Engineering Expectations
Technical Design Template
Pull Request Documentation

[github-actions]

	Warnings
⚠️	PR has more than 500 lines of code changing. Consider splitting into smaller PRs if possible.

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