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)