talkback: mute thumbnail for share screen

src/main/res/layout/share_file_layout.xml

@@ -42,7 +42,7 @@
                 android:layout_gravity="center_vertical"
                 android:layout_marginRight="@dimen/standard_half_margin"
                 android:layout_marginEnd="@dimen/standard_half_margin"
-                android:contentDescription="@string/share_file_icon"
+                android:contentDescription="@null"
                 android:src="@drawable/file"/>
 
             <TextView