Show ripples when day letter is clicked

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

@@ -5,6 +5,7 @@
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:layout_weight="1"
+    android:background="?selectableItemBackgroundBorderless"
     android:gravity="center_horizontal"
     android:textSize="@dimen/normal_text_size"
     tools:text="@string/monday_letter" />