[Fix] [Shared login] Add feature flag to SharedLoginProvider

[RenanLukas]

Adds a feature flag to SharedLoginProvider.

To test:

1 - Install WordPress and login
2 - Set the flag isFeatureFlagEnabled to true (this cannot be done using the Debug settings screen because that is only available when the user is already logged in)
2 - Install Jetpack using the same variant (e.g. if installed WordPress version was jalapenoDebug, install Jetpack jalapenoDebug)
3 - Jetpack should NOT login automatically after launching.

Regression Notes

1. Potential unintended areas of impact
   None

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

3. What automated tests I added (or what prevented me from doing so)
   There is no straightforward way to test the provider with our current testing tools.

PR submission checklist:

• I have completed the Regression Notes.
• 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 these changes on WordPress by downloading wordpress-installable-build-pr17285-08f12e6.apk

	💡 Scan this QR code with your Android phone to download and install the APK directly on it.
	App	WordPress
	Build Flavor	Jalapeno
	Build Type	Debug
	Commit	08f12e6

Note: This installable build uses the JalapenoDebug build flavor, and does not support Google Login.

[wpmobilebot]

📲 You can test these changes on Jetpack by downloading jetpack-installable-build-pr17285-08f12e6.apk

	💡 Scan this QR code with your Android phone to download and install the APK directly on it.
	App	Jetpack
	Build Flavor	Jalapeno
	Build Type	Debug
	Commit	08f12e6

Note: This installable build uses the JalapenoDebug build flavor, and does not support Google Login.

[develric]

Thanks for the PR @RenanLukas 🙇. LGTM 👍

[AliSoftware]

This fix has now been included in the 20.9-rc-3 beta 🎉