add description to FAB for better accessibility

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

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

@@ -89,11 +89,12 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_gravity="bottom|end"
-        android:backgroundTint="@color/colorPrimary"
         android:layout_margin="16dp"
+        android:backgroundTint="@color/colorPrimary"
+        android:contentDescription="@string/nc_new_conversation"
         app:borderWidth="0dp"
-        app:tint="@color/white"
-        app:srcCompat="@drawable/ic_add_white_24px"/>
+        app:srcCompat="@drawable/ic_add_white_24px"
+        app:tint="@color/white" />
 
     <include layout="@layout/fast_scroller" />