use fully opaque ripple mask (#6099)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/1208671518894266/task/1210251609451213?focus=true

### Description
Uses a fully opaque (100% alpha) color for the ripple mask.

The actual color doesn't matter, but devices handle the alpha channel
differently:
- On tested devices, a 0% alpha mask makes the ripple effect invisible.
- On tested Pixels, any alpha > 0% applies the ripple color correctly.
- On tested Samsungs, the ripple alpha is affected by both the mask and
the ripple color alpha, summing them up and causing unexpected results.

To ensure consistent ripple visibility across devices, always use a
fully opaque mask.

Author: LukasPaczos

common/common-ui/src/main/res/drawable/selectable_item_experimental_background.xml

@@ -17,7 +17,18 @@
     android:color="?daxColorControlFillPrimary">
     <item android:id="@android:id/mask">
         <shape android:shape="rectangle">
-            <solid android:color="?attr/daxColorControlFillPrimary" />
+            <!--
+              Use a fully opaque (100% alpha) color for the mask.
+
+              The actual color doesn't matter, but devices handle the alpha channel differently:
+              - On tested devices, a 0% alpha mask makes the ripple effect invisible.
+              - On tested Pixels, any alpha > 0% applies the ripple color correctly.
+              - On tested Samsungs, the ripple alpha is affected by both the mask and the ripple color alpha,
+                summing them up and causing unexpected results.
+
+              To ensure consistent ripple visibility across devices, always use a fully opaque mask.
+            -->
+            <solid android:color="#FF000000" />
             <corners android:radius="14dp" />
         </shape>
     </item>