Move the plugin registrant to io.flutter.plugins and add the @Keep an…

[blasten]

Description

This PR moves the plugin registrant back to the io.flutter.plugins namespace and adds the @keep annotation, so the class isn't removed by R8.

Related Issues

Fixes #43869

Tests

I added the following tests: Updated the unit test.

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• I updated/added relevant documentation (doc comments with ///).
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[matthew-carroll]

LGTM

[blasten]

@RedBrogdon fyi, the examples now need to import io.flutter.plugins.GeneratedPluginRegistrant instead of dev.flutter.plugins.GeneratedPluginRegistrant.