[Analytics Hub] Create shared set of stats helpers for My Store and Analytics Hub

[rachelmcr]

Part of #8149

Description

This PR moves a number of stats data helpers from the StoreStatsPeriodViewModel (used for My Store stats) to a separate StatsV4DataHelper class, so they can be reused in the Analytics Hub.

It also adds a new helper related to the new Average Order Value stat that will be used in Analytics Hub.

Changes

• Moves the shared helpers and unit tests from StoreStatsPeriodViewModel to StatsV4DataHelper.
• Adds a new public helper StatsV4DataHelper.createAverageOrderValueText(orderStats:currencyFormatter:currencyCode:).

Testing

This PR refactors the helpers but there should be no change in the current behavior:

1. Build and run the app.
2. Confirm your stats (text and chart) appear as expected on the My Store screen.
3. Confirm you can select different time period or select specific intervals on the stats chart, and the stats text is updated as expected.

Note: This PR removes some redundant view model unit tests, because the logic is now contained in the helper class. However, you can undo the changes from 466c36f and run the tests in StoreStatsPeriodViewModelTests to confirm that all of the previous tests are still passing with these changes.

Submitter Checklist

Update release notes:

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

[Ecarrion]

Looks good! I wonder if we should start calling this just StatsDataHelper and drop the V4 since it seems like the standard by now!

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8175-d6d6c39 on your iPhone

If you need access to App Center, please ask a maintainer to add you.