UserPersistentStore: UserDefaults Reader & Factory

[alpavanoglu]

Adds the read functions along with a factory that checks a feature flag as fail safe.

To test:

Regression Notes

1. Potential unintended areas of impact N/A

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

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

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.
• [x] 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 pr19170-10a5473 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 pr19170-10a5473 on your iPhone

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