🐛 amp-geo: Target AMP body with class changes (#20946)

[mattwomple]

Use getAmpDoc method to obtain document and body, fixing compatibility
with PWAs, where the body is not simply window.document.body.

Fixes #20946

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[mattwomple]

CLA signed

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CLAs look good, thanks!

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[jpettitt]

LGTM

[jridgewell]

/cc @choumx Does this fit with out plans for AmpDoc?

[mattwomple]

Please do not merge. Issue found: const geoState = doc.getElementById(GEO_ID); will not identify the correct element.

[zhouyx]

@matthewlane Thank you for contributing and submitting the fix! Let's aiming at get #21293 in before the canary cut as well. Thank you

[dreamofabear]

Looks fine to me.