Avoid importing stats multiple times from multiple imports

[garrettr]

Resolves brave/brave-browser#728.

Uses a simple heuristic which should work in the common case: a new brave-browser user wants to import their browser-laptop stats, and may run the import multiple times. The heuristic will fail (and stats import will fail, silently) if the brave-browser user's stats already exceed those from their browser-laptop profile.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.

Test Plan:

1. Import stats from Brave, and open a new tab page to check the current values (should be updated from the import).
2. Import stats from Brave again, then switch back to the new page (it updates automatically). The values for the stats should not have changed.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions

[bbondy]

I originally meant just take the larger of the 2 values, but I think I prefer this way you came up with better. So going with it.