Fix initial visibility of scroll to bottom button.

Fixes #8862

res/layout/conversation_fragment.xml

@@ -43,7 +43,7 @@
 
     <ImageButton
             android:id="@+id/scroll_to_bottom_button"
-            android:visibility="gone"
+            android:visibility="invisible"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:layout_marginEnd="10dp"