Migrate the example to Android embedding v2

[nilsreichardt]

The example is not migrated to Android embedding v2 yet. When running flutter pub get in the example folder, you will get the following warning:

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Warning
──────────────────────────────────────────────────────────────────────────────
Your Flutter application is created using an older version of the Android
embedding. It is being deprecated in favor of Android embedding v2. Follow the
steps at

https://flutter.dev/go/android-project-migration

to migrate your project. You may also pass the --ignore-deprecation flag to
ignore this check and continue with the deprecated v1 embedding. However,
the v1 Android embedding will be removed in future versions of Flutter.
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The detected reason was:

  /Users/nils/Desktop/Projects/Amplitude-Flutter/example/android/app/src/main/AndroidManifest.xml uses
  `android:name="io.flutter.app.FutterApplication"`
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