Use new, consistent assets for adding a bookmark.

Change-Id: I08b492a792e518acb53a69346d51c8c88c06b75e
diff --git a/res/menu/bookmarks.xml b/res/menu/bookmarks.xml
index f768c98..a8781c2 100644
--- a/res/menu/bookmarks.xml
+++ b/res/menu/bookmarks.xml
@@ -16,7 +16,7 @@
 
 <menu xmlns:android="http://schemas.android.com/apk/res/android">
     <item android:id="@+id/new_context_menu_id"
-        android:icon="@android:drawable/ic_menu_add"
+        android:icon="@drawable/ic_menu_add_bookmark"
         android:title="@string/bookmark_page" />
     <item android:id="@+id/switch_mode_menu_id"
         />