Fix post-signup interstitial for login with magic link

[renanferrari]

Fixes #11019

This PR adds a functionality for showing the post-signup interstitial after the login epilogue screen.

In #10976 we tried to completely replace the login epilogue screen for the interstitial screen at the end of the login flow when the user had no sites, to avoid showing an empty epilogue screen. The assumption was that we would have all relevant login flows covered by adding the following logic inside LoginActivity:

WordPress-Android/WordPress/src/main/java/org/wordpress/android/ui/accounts/LoginActivity.java

Lines 245 to 251 in acbf579

	if (!mSiteStore.hasSite() && AppPrefs.shouldShowPostSignupInterstitial()) {
	ActivityLauncher.showPostSignupInterstitial(this);
	} else {
	ActivityLauncher.showMainActivityAndLoginEpilogue(this, oldSitesIds, doLoginUpdate);
	}
	setResult(Activity.RESULT_OK);
	finish();

However, when using a Magic Link, the login epilogue screen is launched from the main activity:

WordPress-Android/WordPress/src/main/java/org/wordpress/android/ui/main/WPMainActivity.java

Lines 1057 to 1058 in fe3fe8b

	ActivityLauncher.showLoginEpilogue(this, true,
	getIntent().getIntegerArrayListExtra(ARG_OLD_SITES_IDS));

This caused the interstitial screen not being shown at all for logins using Magic Link.

A naive approach to this problem would be to replicate the logic above that we used on LoginActivity to the main activity as well, but that wouldn't work as we wanted because, when launched from the main activity, the LoginEpilogueActivity receives an argument doLoginUpdate of true and is responsible to fetch the user account and settings (including sites) itself. This meant we couldn't perform a check to determine if the user had sites, which in turn meant we couldn't determine if it would be possible to replace the empty login epilogue screen for the interstitial screen like we do in the other cases.

After discussing a few options with @maxme, we've decided to go with the solution proposed here, where we'll try to replace the login epilogue screen for the interstitial where possible and let the login epilogue screen be followed by the interstitial where not.

So for example, at the end of the email/password login flow, an user with no sites would not see the login epilogue screen at all. Instead, they would only see the interstitial screen. As for the Magic Link flow, an user with no sites would still see the login epilogue screen, but it would be followed by the interstitial.

This is illustrated below:

Email/Password Flow	Magic Link Flow

To test

1. Clear app data.
2. On the initial screen, tap Log in.
3. Enter an email address that is associated with an account that doesn't have any sites.
4. Complete the login flow using a magic link.
5. On the login epilogue screen, tap Continue.
6. Confirm that the post-signup interstitial screen is shown.

---

PR submission checklist:

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

[maxme]

LGTM,