Fix demo app UnsafeOptInUsageError lint errors

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
(cherry picked from commit 5dbbff4)

demos/main/src/main/java/com/google/android/exoplayer2/demo/SampleChooserActivity.java

@@ -27,6 +27,7 @@
 import android.content.res.AssetManager;
 import android.net.Uri;
 import android.os.AsyncTask;
+import android.os.Build;
 import android.os.Bundle;
 import android.text.TextUtils;
 import android.util.JsonReader;
@@ -271,7 +272,7 @@ private void onSampleDownloadButtonClicked(PlaylistHolder playlistHolder) {
       Toast.makeText(getApplicationContext(), downloadUnsupportedStringId, Toast.LENGTH_LONG)
           .show();
     } else if (!notificationPermissionToastShown
-        && Util.SDK_INT >= 33
+        && Build.VERSION.SDK_INT >= 33
         && checkSelfPermission(Api33.getPostNotificationPermissionString())
             != PackageManager.PERMISSION_GRANTED) {
       downloadMediaItemWaitingForNotificationPermission = playlistHolder.mediaItems.get(0);