Store & use ConvDev percentages returned by Version app

What does this MR do?

It stores the percentages values returned by the Version app and uses the stores values instead of calculating them again (if the stored values exist)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We decided to calculate metric percentages only at one place - in the Version app (https://gitlab.com/gitlab-com/salesforce/issues/169#note_35982164) and we return these calculated values as the response to to the GitlabUsagePingWorker.

We need to store these returned values and use them instead of calculating them again on the GitLab side.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #35761 (closed)