Declare demo service as MediaLibraryService

The `PlaybackService` of the demo app is declared
as `androidx.media3.session.MediaSessionService` instead
of `androidx.media3.session.MediaLibraryService`. While
this technically works, its confusing to do that in the
demo app.

Generally, apps that declare the legacy
`android.media.browse.MediaBrowserService` should also
declare `androidx.media3.session.MediaLibraryService`
and the demo app should reflect this common case.

Issue: #672
PiperOrigin-RevId: 595320994

demos/session/src/main/AndroidManifest.xml

@@ -59,7 +59,7 @@
         android:foregroundServiceType="mediaPlayback"
         android:exported="true">
       <intent-filter>
-        <action android:name="androidx.media3.session.MediaSessionService"/>
+        <action android:name="androidx.media3.session.MediaLibraryService"/>
         <action android:name="android.media.browse.MediaBrowserService"/>
         <action android:name="android.media.action.MEDIA_PLAY_FROM_SEARCH"/>
       </intent-filter>