Jetpack app: Enable account creation

[ashiagr]

Closes #15922

This PR enables signup using WordPress.com in the Jetpack app using the ENABLE_SIGNUP constant.

To test:

1. Install and login to the Jetpack app.
2. Notice that Log in or signup with WordPress.com is shown on the Login Prologue screen.
3. Signup using a new email.
4. Notice that signup succeeds.

Merging Notes:

• Don't merge the PR yet, I'll take care of the merging.

Regression Notes

1. Potential unintended areas of impact
   WordPress app flow.

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

3. What automated tests I added (or what prevented me from doing so)
   Updated exiting unit tests for Jetpack app Login Prologue screen.

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.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15928-build-118825.apk

• WordPress-pr-15928-build-118825.apk

[ashiagr]

Reviewed as part of feature branch PR #15946.