Do not show Snackbar message for automatically added items

When the main page becomes visible on the screen, 4 items are automatically added to the seqeuence(s). With each added item, we present a snackbar message to signal adding it.
However, the snackbar message doesn't matter much for these items, and we'd need to wait for them to be disappear before we click on preview.

PiperOrigin-RevId: 781091774

Author: shahdDaghash

demos/composition/src/main/java/androidx/media3/demo/composition/CompositionPreviewViewModel.kt

@@ -121,11 +121,11 @@ class CompositionPreviewViewModel(application: Application, val compositionLayou
     for (i in titles.indices) {
       mediaItemOptions.add(Item(titles[i], uris[i], durations[i].toLong(), mutableStateOf(false)))
     }
-    // Load initial media item selections
-    addItem(0)
-    addItem(0)
-    addItem(0)
-    addItem(0)
+    // Load initial media item selections. No need to show the Snackbar message at this point
+    addItem(0, showSnackbarMessage = false)
+    addItem(0, showSnackbarMessage = false)
+    addItem(0, showSnackbarMessage = false)
+    addItem(0, showSnackbarMessage = false)
   }
 
   override fun onCleared() {
@@ -140,9 +140,11 @@ class CompositionPreviewViewModel(application: Application, val compositionLayou
     DebugTraceUtil.enableTracing = enable
   }
 
-  fun addItem(index: Int) {
+  fun addItem(index: Int, showSnackbarMessage: Boolean = true) {
     selectedMediaItems.add(mediaItemOptions[index].copy(applyEffects = mutableStateOf(false)))
-    snackbarMessage = "Added item: ${mediaItemOptions[index].title}"
+    if (showSnackbarMessage) {
+      snackbarMessage = "Added item: ${mediaItemOptions[index].title}"
+    }
   }
 
   fun removeItem(index: Int) {