Added cancellation check for MediaBrowserFuture in demo session app

When app is deployed with device's screen being off, MainActivity's onStart is called swiftly by its onStop. The onStop method cancels the browserFuture task which in turn "completes" the task. Upon task "completion", pushRoot() runs and then throws error as it calls get() a cancelled task.

PiperOrigin-RevId: 492416445

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

@@ -38,7 +38,7 @@ import com.google.common.util.concurrent.ListenableFuture
 class MainActivity : AppCompatActivity() {
   private lateinit var browserFuture: ListenableFuture<MediaBrowser>
   private val browser: MediaBrowser?
-    get() = if (browserFuture.isDone) browserFuture.get() else null
+    get() = if (browserFuture.isDone && !browserFuture.isCancelled) browserFuture.get() else null
 
   private lateinit var mediaListAdapter: FolderMediaItemArrayAdapter
   private lateinit var mediaListView: ListView