ensure that tab switcher grid items always scale images and uses square page previews in the old design (#6529)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1208671518894266/task/1210976600737969?focus=true

### Description
Fixes a regression from #6521
which unintentionally removed scaling from placeholder images in the tab
switcher grid items.

### Steps to test this PR
- [x] Open the tab switcher while on new tab page and verify that the
Dax icon is the right scale.

### UI changes
| Before  | After |
| ------ | ----- |
<img width="960" height="2142" alt="Screenshot_20250805_163405"
src="https://github.com/user-attachments/assets/abd14c30-5c85-452c-8647-71531c7685aa"
/>|<img width="960" height="2142" alt="Screenshot_20250805_163432"
src="https://github.com/user-attachments/assets/88115fb5-a0f1-4c21-a6e6-32b52319d75c"
/>|

Author: LukasPaczos

app/src/main/java/com/duckduckgo/app/tabs/ui/TabSwitcherAdapter.kt

@@ -403,10 +403,18 @@ class TabSwitcherAdapter(
                 try {
                     glide.load(cachedWebViewPreview)
                         .transition(DrawableTransitionOptions.withCrossFade())
-                        .transform(
-                            fitAndClipBottom(),
-                            RoundedCorners(tabPreview.context.resources.getDimensionPixelSize(CommonR.dimen.smallShapeCornerRadius)),
-                        )
+                        .let {
+                            if (isVisualExperimentEnabled) {
+                                it.transform(
+                                    fitAndClipBottom(),
+                                    RoundedCorners(tabPreview.context.resources.getDimensionPixelSize(CommonR.dimen.smallShapeCornerRadius)),
+                                )
+                            } else {
+                                it.transform(
+                                    fitAndClipBottom(),
+                                )
+                            }
+                        }
                         .into(tabPreview)
                 } catch (e: Exception) {
                     logcat(ERROR) { "Error loading tab preview for ${tab.tabId}: ${e.message}" }

app/src/main/res/layout/item_tab_grid_new.xml

@@ -100,6 +100,7 @@
             android:layout_height="@dimen/gridItemPreviewHeightNew"
             android:layout_marginTop="6dp"
             android:importantForAccessibility="no"
+            android:scaleType="center"
             app:layout_constraintBottom_toBottomOf="parent"
             app:layout_constraintEnd_toEndOf="parent"
             app:layout_constraintStart_toStartOf="parent"