Add talk back for regulatory info

Bug: 30077425
Test: Manual -
1. Switch on talk back and click "Regulatory labels" in "About phone".
2. Click the regulatory image.
3. Check whether it has talk back.

Change-Id: Ia06d2d932f13a6973dfbd4899dcb17c67ae670f6

res/layout/regulatory_info.xml

@@ -22,6 +22,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:scaleType="centerCrop"
+        android:contentDescription="@string/regulatory_labels"
         android:src="@drawable/regulatory_info" />
 </ScrollView>