This change:
* Adds missing `@OptIn` annotation to demo app's `ErrorMessageProvider`
* Switches from `Util.SDK_INT` to `Build.VERSION.SDK_INT` in
`SampleChooserActivity` (`PlayerActivity` is already using this).
This code hasn't changed recently, and it doesn't fail on the `release`
branch, but it failed when I checked the `main` branch just now - so I
assume lint has updated to detect more cases where unstable APIs are
being used without opt-in. I suspect the difference is due to different
Android Gradle Plugin versions between the branches.
#minor-release
PiperOrigin-RevId: 529111669
