Use a for-each loop instead of forEach in PlaybackService.kt

The current code flags a lint error:

```
Error: Call requires API level 24 (current min is 16): java.lang.Iterable#forEach [NewApi]
```

I think this is a bit confusing because this is calling the Java
[`Iterable.forEach`](https://developer.android.com/reference/java/lang/Iterable#forEach(java.util.function.Consumer%3C?%20super%20T%3E))
method which was added in Java 8 (and therefore is only available on
API 24 and up), but there is **also** a Kotlin
[`List.forEach`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/for-each.html)
method which is available in all versions of Kotlin (and therefore all
Android versions). Since this is a Kotlin file, at first glance you
would assume this is the Kotlin method - but it's not.

This also doesn't seem to be flagged by Android Studio, but is caught
by Gradle lint on the command line.

#minor-release

PiperOrigin-RevId: 529112610
(cherry picked from commit 09b474a519)

demos/session/src/main/java/androidx/media3/demo/session/PlaybackService.kt

@@ -96,7 +96,7 @@ class PlaybackService : MediaLibraryService() {
     ): MediaSession.ConnectionResult {
       val connectionResult = super.onConnect(session, controller)
       val availableSessionCommands = connectionResult.availableSessionCommands.buildUpon()
-      customCommands.forEach { commandButton ->
+      for (commandButton in customCommands) {
         // Add custom command to available session commands.
         commandButton.sessionCommand?.let { availableSessionCommands.add(it) }
       }