Work around change in SimpleXMLElement behavior introduced in PHP 7.3.17

Work around change in SimpleXMLElement behavior introduced in PHP 7.3.17

Upstream bug reports of the behavior change introduced in PHP 7.3.17 (and
applied to PHP 7.4 branch as well):
https://bugs.php.net/bug.php?id=79528
https://bugs.php.net/bug.php?id=79485

The reponsible commit in PHP was https://github.com/php/php-src/pull/5246

This was a "bug fix" in the sense that SimpleXML used to discard the
attributes on the namespace elements, which look like this:
     <namespace key="-2" case="first-letter">Media</namespace>
SimpleXML used to return this as a string "Media" instead of a
SimpleXMLElement... but ExportTest (inadvertently?) depended on that
behavior.

In any case, if we iterate over SimpleXMLElement::children() we always
get SimpleXMLElements, not "sometimes strings", and so our code will
correct correctly on PHP below 7.3.17 and above, regardless of how PHP
decides to handle this "bug".

Bug: T250568
Change-Id: I9c2cb6a86fd6e8023c1979ec6838071a87a7bcea
(cherry picked from commit 7f1ad7d9848782d025bad63149e058964fc37c97)