Add string resource to puf-ondatachange (#507)

Author: SUPERCILEX

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

@@ -10,7 +10,7 @@
         android:id="@+id/emptyTextView"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:text="No messages. Start chatting at the bottom!"
+        android:text="@string/start_chatting"
         android:padding="16dp" />
 
     <android.support.v7.widget.RecyclerView

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

@@ -64,4 +64,7 @@
     <string name="choose_image">Choose Image</string>
     <string name="accessibility_downloaded_image">Downloaded image</string>
     <string name="drive_file">Drive File</string>
+
+    <!-- strings for Auth UI demo activities -->
+    <string name="start_chatting">No messages. Start chatting at the bottom!</string>
 </resources>