add content description to preview image

src/main/res/layout/toolbar_standard.xml

@@ -34,6 +34,7 @@
             android:id="@+id/preview_image"
             android:layout_width="match_parent"
             android:layout_height="@dimen/nav_drawer_header_height"
+            android:contentDescription="@string/preview_image_description"
             android:scaleType="centerCrop"
             android:visibility="gone" />