refactor: migrate WidgetsBindingObserver to AppLifecycleListener

[S-ecki]

As noted in a todo comment from filiph, listening to the lifecycle events should be migrated to the new AppLifecycleListener according to this blog post.

This has been done for all 3 templates and all 2 samples.

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• I read the Flutter Style Guide recently, and have followed its advice.
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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

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[domesticmouse]

PTAL @filiph