XhtmlStringIndexer does not find references inside personalized xhtmlstrings

Description

Steps to reproduce:

1. Install an Alloy site with tinymce.
2. Add a new block and publish it.
3. Add a page which has an XHtmlString (for example, a News page with has an XHtmlString mainbody).
4. Add the block to the XHtmlString field.
5. Publish the page.
6. Open the block and verify that it says "Changes made here will affect at least 1 item".
7. Open the page and add a personalization to the block in the XHtmlString.
8. Publish the page.
9. Open the block.

Expected:
Should say "Changes made here will affect at least 1 item"

Actual:
Says "This item is not used anywhere."