[fenix] For https://github.com/mozilla-mobile/fenix/issues/16323 - Use 10dp corner radius for the selected outer border of a grid item (https://github.com/mozilla-mobile/fenix/pull/16364)

This would ensure the corners of the inner and outer borders are aligned, with
no empty gap between them.

app/src/main/res/drawable/tab_tray_grid_item_selected_border.xml

@@ -19,7 +19,7 @@
                 android:width="4dp"
                 android:color="@color/photonViolet70" />
 
-            <corners android:radius="@dimen/tab_tray_grid_item_border_radius" />
+            <corners android:radius="@dimen/tab_tray_grid_item_outer_border_radius" />
         </shape>
 
     </item>

app/src/main/res/values/dimens.xml

@@ -179,6 +179,7 @@
     <dimen name="tab_tray_grid_item_thumbnail_width">156dp</dimen>
     <dimen name="tab_tray_grid_item_thumbnail_height">156dp</dimen>
     <dimen name="tab_tray_grid_item_border_radius">8dp</dimen>
+    <dimen name="tab_tray_grid_item_outer_border_radius">10dp</dimen>
     <dimen name="tab_tray_grid_item_selected_border_width">2dp</dimen>
     <dimen name="tab_tray_favicon_border_radius">4dp</dimen>
     <dimen name="tab_tray_multiselect_handle_height">11dp</dimen>