Add post-signup interstitial feature

[renanferrari]

Master issue: #10918

Related tasks:

• Create interstitial screen: issue | pull request
• Display interstitial screen: issue | pull request
• Track interstitial screen events: issue | pull request

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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.

[peril-wordpress-mobile]

	Warnings
⚠️	PR has more than 500 lines of code changing. Consider splitting into smaller PRs if possible.

Messages

📖

This PR contains changes in the subtree libs/mocks/. It is your responsibility to ensure these changes are merged back into wordpress-mobile/WordPressMocks. Follow these handy steps! WARNING: Make sure your git version is 2.19.x or lower - there is currently a bug in later versions that will corrupt the subtree history! cd WordPress-Android git checkout feature/10918-post-signup-interstitial git subtree push --prefix=libs/mocks/ https://github.com/wordpress-mobile/WordPressMocks.git merge/WordPress-Android/11024 Browse to https://github.com/wordpress-mobile/WordPressMocks/pull/new/merge/WordPress-Android/11024 and open a new PR.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[maxme]

We're merging the feature branch feature/10918-post-signup-interstitial where all smaller PRs were reviewed. I did some more manual tests and had a look at our analytic tools to make sure we tracked the correct events.