Use artwork display mode in demo app

- Use artwork display mode `fill` to improve visual apperance
- Some minor cleanup

#minor-release

PiperOrigin-RevId: 534366246

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

@@ -88,8 +88,7 @@ class PlayableFolderActivity : AppCompatActivity() {
       browser.shuffleModeEnabled = true
       browser.prepare()
       browser.play()
-      val intent = Intent(this, PlayerActivity::class.java)
-      startActivity(intent)
+      browser?.sessionActivity?.send()
     }
 
     findViewById<Button>(R.id.play_button).setOnClickListener {

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

@@ -298,8 +298,6 @@ class PlaybackService : MediaLibraryService() {
       val pendingIntent =
         TaskStackBuilder.create(this@PlaybackService).run {
           addNextIntent(Intent(this@PlaybackService, MainActivity::class.java))
-
-          val immutableFlag = if (Build.VERSION.SDK_INT >= 23) FLAG_IMMUTABLE else 0
           getPendingIntent(0, immutableFlag or FLAG_UPDATE_CURRENT)
         }
       val builder =

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

@@ -122,7 +122,7 @@ class PlayerActivity : AppCompatActivity() {
   }
 
   private fun updateMediaMetadataUI(mediaMetadata: MediaMetadata) {
-    val title: CharSequence = mediaMetadata.title ?: "getString(R.string.no_item_prompt)"
+    val title: CharSequence = mediaMetadata.title ?: ""
 
     findViewById<TextView>(R.id.media_title).text = title
     findViewById<TextView>(R.id.media_artist).text = mediaMetadata.artist

demos/session/src/main/res/layout/activity_player.xml

@@ -31,6 +31,7 @@
       android:background="@color/player_background"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
+      app:artwork_display_mode="fill"
       app:default_artwork="@drawable/artwork_placeholder"
       app:repeat_toggle_modes="one|all"
       app:show_shuffle_button="true"