[file_selector_android] Remove deprecated usesCleartextTraffic

[mboetger]

Removes deprecated usesCleartextTraffic and replaces it with a network configuration policy file.

Addresses: #182553

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[flutter-dashboard]

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[gemini-code-assist]

Code Review

This pull request removes the android:usesCleartextTraffic="true" attribute from the <application> tag in the packages/file_selector/file_selector_android/example/android/app/src/debug/AndroidManifest.xml file. This change addresses the deprecation of the attribute and enhances security by preventing cleartext traffic, which is appropriate as the file selector functionality does not require it.