Add missing initial update to the subtitle button in the session demo

The button gets only updated after an onTracksChanged callback but
isn't set to the initial state similar to other changes triggered
by callbacks (e.g. updateRepeatSwitchUI)

PiperOrigin-RevId: 522274800

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

@@ -135,6 +135,7 @@ class PlayerActivity : AppCompatActivity() {
     updateMediaMetadataUI(controller.mediaMetadata)
     updateMediaMetadataUI(controller.mediaMetadata)
     updateShuffleSwitchUI(controller.shuffleModeEnabled)
     updateShuffleSwitchUI(controller.shuffleModeEnabled)
     updateRepeatSwitchUI(controller.repeatMode)
     updateRepeatSwitchUI(controller.repeatMode)
+    playerView.setShowSubtitleButton(controller.currentTracks.isTypeSupported(TRACK_TYPE_TEXT))
 
 
     controller.addListener(
     controller.addListener(
       object : Player.Listener {
       object : Player.Listener {