Allow MediaLibraryService to reject the resumption notification

To reliably reject the System UI playback resumption notification on
all API levels (specifically API 30), the backward compatibility layer
needs to return `null` for the library root.

This is not possible in the Media3 implementation. This change allows
an app to return a `LibraryResult.ofError(RESULT_ERROR_NOT_SUPPORTED)`
that then is translated to return null by the backwards compatibility
layer.

Issue: androidx/media#355
Issue: androidx/media#167
Issue: androidx/media#27

See https://developer.android.com/guide/topics/media/media-controls#mediabrowserservice_implementation

PiperOrigin-RevId: 527276529

RELEASENOTES.md

@@ -56,6 +56,10 @@
     *   Add helper method `MediaSession.getControllerForCurrentRequest` to
         obtain information about the controller that is currently calling
         a`Player` method.
+    *   Fix bug that prevented the `MediaLibraryService` from returning null for
+        a call from System UI to `Callback.onGetLibraryRoot` with
+        `params.isRecent == true` on API 30
+        ([#355](https://github.com/androidx/media/issues/355)).
 *   UI:
 
     *   Add Util methods `shouldShowPlayButton` and

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

@@ -30,6 +30,7 @@ import androidx.media3.common.util.Util
 import androidx.media3.datasource.DataSourceBitmapLoader
 import androidx.media3.exoplayer.ExoPlayer
 import androidx.media3.session.*
+import androidx.media3.session.LibraryResult.RESULT_ERROR_NOT_SUPPORTED
 import androidx.media3.session.MediaSession.ControllerInfo
 import com.google.common.collect.ImmutableList
 import com.google.common.util.concurrent.Futures
@@ -143,6 +144,12 @@ class PlaybackService : MediaLibraryService() {
       browser: ControllerInfo,
       params: LibraryParams?
     ): ListenableFuture<LibraryResult<MediaItem>> {
+      if (params != null && params.isRecent) {
+        // The service currently does not support playback resumption. Tell System UI by returning
+        // an error of type 'RESULT_ERROR_NOT_SUPPORTED' for a `params.isRecent` request. See
+        // https://github.com/androidx/media/issues/355
+        return Futures.immediateFuture(LibraryResult.ofError(RESULT_ERROR_NOT_SUPPORTED))
+      }
       return Futures.immediateFuture(LibraryResult.ofItem(MediaItemTree.getRootItem(), params))
     }
 

libraries/session/src/main/java/androidx/media3/session/MediaLibraryServiceLegacyStub.java

@@ -126,8 +126,11 @@ public BrowserRoot onGetRoot(
           .putBoolean(BROWSER_SERVICE_EXTRAS_KEY_SEARCH_SUPPORTED, isSearchSessionCommandAvailable);
       return new BrowserRoot(result.value.mediaId, extras);
     }
-    // No library root, but keep browser compat connected to allow getting session.
-    return MediaUtils.defaultBrowserRoot;
+    // No library root, but keep browser compat connected to allow getting session unless the
+    // `Callback` implementation has not returned a `RESULT_SUCCESS`.
+    return result != null && result.resultCode != RESULT_SUCCESS
+        ? null
+        : MediaUtils.defaultBrowserRoot;
   }
 
   // TODO(b/192455639): Optimize potential multiple calls of