Jetpack Focus: Enable "What Is New" for Jetpack app

[momo-ozawa]

Part of #19164

Description

• Updated AppConfiguration.showsWhatIsNew to true for Jetpack
• Extracted the "What's New in " string to AppConstants

How to test

TBD

Regression Notes

1. Potential unintended areas of impact

• What's new on WP

2. What I did to test those areas of impact (or what existing automated tests I relied on)

• Tested the steps above

3. What automated tests I added (or what prevented me from doing so)

• n/a

PR submission checklist:

• [x] I have completed the Regression Notes.
• [x] I have considered adding unit tests for my changes.
• [x] I have considered adding accessibility improvements for my changes.
• [ ] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19176-c705a71 on your iPhone

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

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19176-c705a71 on your iPhone

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

[momo-ozawa]

@enejb @hassaanelgarem @Gio2018

I have two test announcements enabled for this PR on the mobile announce mc tool:

• JPiOS: 235
• WPiOS: 188

I'll disable these once this PR is approved.