WaitingTimeTracker improvements

[iamgabrielma]

Description

This PR is a side-quest from #15047 to improve WaitingTimeTracker's readability. In that PR we reused the existing WaitingTimeTracker to capture POS events in milliseconds without realizing the actual elapsed time was being captured in seconds. With these changes we extend the class to allow both, and be more explicit about it.

Changes:

• Updates ...InMillis property in the interface to avoid using it incorrectly by mistake, by being explicit about the time unit.
• Updates internals to use seconds (by default) or milliseconds by choice
• Adds additional tests

Testing information

• Pre-existing WooAnalyticsEvent.WaitingTime.Scenario are still tracked in seconds with the key waiting_time. For example these two run as soon as we load the app:

🔵 Tracked dashboard_main_stats_waiting_time_loaded, properties: [store_id: c5bd46cc-1804-4f7b-badb-bb98c449127f, is_wpcom_store: false, blog_id: -1, plan: , site_url: https://indiemelon.mystagingwebsite.com, waiting_time: 2.065809965133667, was_ecommerce_trial: false]
🔵 Tracked dashboard_top_performers_waiting_time_loaded, properties: [store_id: c5bd46cc-1804-4f7b-badb-bb98c449127f, plan: , site_url: https://indiemelon.mystagingwebsite.com, was_ecommerce_trial: false, is_wpcom_store: false, blog_id: -1, waiting_time: 7.46925497055054]

• In a POS eligible-store, load POS and observe the loading event is tracked in milliseconds with the key milliseconds_time_elapsed_in_splash_screen:

🔵 Tracked pos_pos_loaded, properties: [is_wpcom_store: false, blog_id: -1, milliseconds_time_elapsed_in_splash_screen: 2152.8499126434326, site_url: https://indiemelon.mystagingwebsite.com, was_ecommerce_trial: false, plan: , store_id: c5bd46cc-1804-4f7b-badb-bb98c449127f]

Tested in simulator iPad Air 11 iOS 17.5

---

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[dangermattic]

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[iamgabrielma]

Only one review is needed. cc: @ThomazFB in case I missed something here 👀

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr15082-6de1ef0
	Version	21.6
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	6de1ef0
	App Center Build	WooCommerce - Prototype Builds #12844

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