Fix selection of Reader theme causing validated URLs to always be stale

[westonruter]

Summary

I've selected Reader mode and chose a Reader theme:

Upon doing a Site Scan, the stale message goes away. But after I reload, the stale message appears:

Going to the AMP Validated URLs screen shows that all the just-scanned URLs are stale:

My active theme is twentyseventeen and my Reader theme is twentythirteen, and I can see that AMP_Validated_URL_Post_Type::get_post_staleness() is returning unexpectedly:

{
    "theme": {
        "new": [
            "twentyseventeen"
        ],
        "old": [
            "twentythirteen"
        ]
    }
}

Clearly \AMP_Validated_URL_Post_Type::get_validated_environment() is not taking Reader themes into account. This fixes that.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

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[dhaval-parekh]

Changes look good to me. ✅

QA Passed
It no longer shows stale results after reloading the page after site scanning when the reader theme is selected.

[delawski]

Works as expected for me, too. After doing a Site Scan in the Reader mode and then reloading the page, I get the "No changes since your last scan" message which is correct: