[google_sign_in] Remove usersCleartextTraffic

[mboetger]

Removes deprecated usesCleartextTraffic.

Addresses: #182553

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[gemini-code-assist]

Code Review

This pull request removes the android:usesCleartextTraffic attribute from the example app's debug AndroidManifest.xml. It also removes the espresso development dependency from the example's pubspec.yaml.

[stuartmorgan-g]

LGTM

[stuartmorgan-g]

version/changelog override: dev dependency changes don't affect clients.