optimize for rtl/ltr and improve a11y

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

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

@@ -114,15 +114,16 @@
             android:id="@+id/scrollDownButton"
             android:layout_width="44dp"
             android:layout_height="44dp"
-            android:layout_alignParentRight="true"
+            android:layout_alignParentEnd="true"
             android:layout_alignParentBottom="true"
             android:layout_marginEnd="6dp"
             android:layout_marginBottom="16dp"
             android:background="@drawable/shape_oval"
             android:backgroundTint="@color/scroll_down_chat_button"
+            android:contentDescription="@string/scroll_to_bottom"
             android:src="@drawable/ic_baseline_keyboard_double_arrow_down_24"
-            app:tint="@color/scroll_down_chat_button_icon"
-            android:visibility="invisible" />
+            android:visibility="invisible"
+            app:tint="@color/scroll_down_chat_button_icon" />
 
     </RelativeLayout>
 

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

@@ -630,5 +630,6 @@
 
     <string name="nc_not_allowed_to_activate_audio">You are not allowed to activate audio!</string>
     <string name="nc_not_allowed_to_activate_video">You are not allowed to activate video!</string>
+    <string name="scroll_to_bottom">Scroll to bottom</string>
 
 </resources>