Add proper background color to group replies.

app/src/main/res/drawable/rounded_rectangle_surface_variant_18.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle" >
+    <solid android:color="@color/signal_colorSurfaceVariant"/>
+    <corners android:radius="18dp"/>
+</shape>

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

@@ -63,7 +63,7 @@
         android:id="@+id/bubble"
         android:layout_width="0dp"
         android:layout_height="0dp"
-        android:background="@drawable/rounded_rectangle_secondary_18"
+        android:background="@drawable/rounded_rectangle_surface_variant_18"
         app:layout_constraintBottom_toBottomOf="parent"
         app:layout_constraintEnd_toEndOf="@+id/bubble_end_barrier"
         app:layout_constraintStart_toStartOf="@+id/bubble_start_barrier"