refactor: update desktop windowing snippets

* Wrap transparent action bar logic in `TransparentActionBarActivity`
* Convert drag-and-drop source and target extension functions to standard functions

Author: prateekbatra-g

compose/snippets/src/main/java/com/example/compose/snippets/adaptivelayouts/DesktopWindowing.kt

@@ -93,14 +93,16 @@ class TransparentActionBarActivity : Activity() {
 /**
  * Simple drag source for plain text data.
  */
-// [START android_compose_desktop_drag_drop_source]
-fun Modifier.dragAndDropSourceModifier(): Modifier = this.dragAndDropSource { _ ->
-    DragAndDropTransferData(
-        clipData = ClipData.newPlainText("label", "Your data"),
-        flags = View.DRAG_FLAG_GLOBAL_SAME_APPLICATION
-    )
+fun dragAndDropSourceModifier() {
+    // [START android_compose_desktop_drag_drop_source]
+    Modifier.dragAndDropSource { _ ->
+        DragAndDropTransferData(
+            clipData = ClipData.newPlainText("label", "Your data"),
+            flags = View.DRAG_FLAG_GLOBAL_SAME_APPLICATION
+        )
+    }
+    // [END android_compose_desktop_drag_drop_source]
 }
-// [END android_compose_desktop_drag_drop_source]
 
 /**
  * Custom drag source that launches a new Activity instance when dropped.